By SETH KUGEL                                                                                                                             

T 5 o`clock on a hot Sunday afternoon in early December, a few scattered loungers chatted over draft beers on the verandas of the Rua dos Oitis, a graceful, narrow street of informal restaurant-bars in Rio`s affluent neighborhood of Baixo Gávea.

Under the Brazilian version of daylight saving time, the summer sun had several hours left to dapple the adjoining plaza, the Praça Santos Dumont, where a few old men dotted benches and a young couple admired the fountain. A bustling thoroughfare on the other side of the trees seemed a mile away. Occasionally an overzealous puppy yipped, interrupting the quiet, or a taxi dropped off a passenger at one of the nearby apartment buildings.

The calm was unremarkable: most of Rio de Janeiro was, as usual on summer weekends, at the beach.

By 7:30, signs began appearing that a party was about to materialize. In couples and groups, advance units of the perma-bronzed, perma-happy après-beach crowd drifted in - parking their Peugeots or Volkswagen Golfs on nearby streets to be attended for the evening by men working for tips - and made their way into the bars.

By 10, the Rua dos Oitis was a happy hour run amok. Throngs spilled out of the Hipódromo Up, popular for its pizza, including the multitopped portuguêsa version. Women in jeans shorts and skimpy tops smiled at men buying them Skol beer from coolers in the crowded street. The seductive buzz of Brazilian Portuguese was punctuated by the shouts of friends hailing one another as they pushed through the congestion to bestow double-cheeked kisses. Despite their evident delight at meeting, many had probably shared a patch of sand a few hours earlier.

My cellphone buzzed; Carolina Barreto, one of my new local circle of acquaintances, was trying to find me in the crowd. She and her friends were only 50 feet away, but it took five minutes to get to them.

It was all typically, intangibly Rio - tanned midriffs and intoxicating smiles, informal celebration and friendly exchanges, the casual democracy of an American street festival combined with the charged vibe of a South Beach nightclub.

This was not precisely a planned event: The Rua dos Oitis fills up regularly on Thursdays and Sundays by a kind of unwritten common consent. Cariocas, as the people of Rio de Janeiro call themselves, have a magical tendency to know exactly where the next high-density, low-pressure social gathering is going to appear.

Sometimes it will be in a small club with spellbinding local music, sometimes in a shopping mall soaked in beer and cachaça, the signature sugar cane liquor of Brazil. Sometimes, too, it will be on an iconic urban beach with kiosks selling cold coconut water for 50 cents.

Americans may think of the beach as literally a laid-back affair, but on the sands of Barra da Tijuca or Ipanema, social butterflies outnumber book readers 10 to 1, and small stretches of oceanfront morph into packed-in party spots. So many people seem to know each other that remembering to flip over to even out your tan is a nonissue, rendered irrelevant by how often you are roused by the arrival of yet another friend in yet another impossibly tiny bathing suit.

The street gatherings at Baixo Gávea used to happen on Mondays instead of Thursdays and Sundays (Segunda-sem-lei, or Lawless Monday, it was called), until the complaints of well-organized party-pooping neighbors won out. Alas, in a city of 10.8 million, not everyone can be young, beautiful and available to party any night of the week.

Most tourists in Rio spend most of their time downtown or in the city`s Zona Sul, or southern zone, where the Rua dos Oitis is located. But in the 50 weeks of the year not devoted to Carnaval or New Year`s Eve, it can be easy to miss the party. It takes some guidance to develop the sense of where the Cariocas will be exercising their native joie de vivre.

A working knowledge of Portuguese is an easy in, but even lacking that, with a little advance work and a few English-speaking Brazilian contacts you can weave your way into the action and get a glimpse of the real scene.

I received my initial orientation at home in New York, from acquaintances and friends of friends. Many Brazilians, gregarious by nature, are happy enough to help steer a traveler, especially if they think they may be coming north sometime to collect on a return of the favor. Local advice is also comforting, of course, given Rio`s reputation for crime. While the danger does not seem to dampen anyone`s partying spirit, violence is much feared and the threat is much discussed among the locals.

Frequent travelers to Rio may share tips, too. Before and after its peak travel season (it is past now, since Carnaval ended Feb. 8), the city attracts a core of regular visitors wooed back by the charm of the Brazilians and the culture of their proudest city. These repeaters readily brave long flights, like the 12-hour trip from New York with not a single nonstop to be found.

They come because the physical environment, with its dramatic rock formations and lovely beaches somehow plunked down in a huge city, is famously unbeatable, and because this is a worldly city; Ipanema Beach, for example, is just a few blocks off a sophisticated shopping street with everything from bookstores to banks to bikini shops.

The city`s beauty lies mostly in its setting and its people, but here and there are gems of colonial and neo-Classical style architecture, and charming neighborhoods like the hillside Santa Teresa, where an ancient tram provides access to homey but hip restaurants and enchanting gift shops featuring works by local artists.

And there are local touches like the ubiquitous juice-and-sandwich stands. You have to love a country where menus list so many translation-defying fruits - graviola, fruta de conde, açaí - that mangoes seem downright unexotic.

The core of Rio lovers come in all wallet sizes; they are distinguishable by their desire to visit and keep visiting, and to stay as long as possible. Eric Linder, a 25-year-old from Los Angeles who was the only other American I found in Baixo Gávea that night, had first arrived with images of a hedonistic paradise, but those had faded: now he was simply a night-life addict.

"Bars in L. A. close at 2," he said. "Here they stay open until sunrise. But the best thing for me is, it`s casual. You don`t have to get dressed up to have a few beers." He had decided to stay until his money ran out, and so far he had made it last a month.

Some of the better heeled go further: they buy a place. David Parker, a 51-year-old magazine consultant from New York, closed on a duplex penthouse in the Copacabana neighborhood last May after several monthlong trips to Rio. The city for him is not merely a vacation destination, it is a second home made possible by telecommuting.

"There`s a commitment you make to Rio," he told me. "If you`re spending more than one trip, it`s because you`ve fallen in love with the people and the culture and the way of life. It`s not just because this is a fun place to party for the weekend."

Nevertheless, if you want to find the Cariocas, come equipped with some party spirit.

At 10:30 on a Friday night in December, Rosa Shopping, a mundane outdoor shopping center off a highway of American-style malls and Outback Steakhouses, teemed with Carioca men dressed in locally popular Osklen surf wear and women in brightly colored jewelry and in skirts and blouses from stores such as Farm, flirting and balancing drinks. A guitarist played outside of the Nó de Corda bar, where the cachaça flowed com mel - with a dollop of honey in the shot glass, ready to impart a sugar high along with the liquor jolt.

It appeared that the entire young population of Barra da Tijuca, this mall-ridden area of the city, was concentrated on stone walkways around a parklike courtyard, but I was assured that the place was empty. I had arrived too late; the peak crowd was mostly en route to the evening`s real destinations - nightclubs and private parties.

Sky Lounge, possibly a final destination for some in the crowd that night, was similarly unimpressive at first glance - just a comfortable space with skylights, sparely decorated with cool, low-slung sofas and chairs. But it is one of the hot clubs this summer, spoken of with reverence (if oddly pronounced as e-SKEE LAWN-gee) by just about everyone I encountered.

It is not cheap: drinks like the apple and cinnamon infusion go for a locally outrageous $5 (in American dollars), but that does not keep away the crowds, especially on weekend nights. Samba-free American dance music most nights puts gringos on an equal footing, for once, with the overwhelmingly Carioca crowd.

In the wee hours of Monday, half the crowd was making out with boyfriend, girlfriend, friend-friend or possibly just a random stranger, recalling a passage from "Malu de Bicicleta," a novel by the Brazilian author Marcelo Rubens Paiva: "It was that phase of the party when kissing on the mouth was just like asking for a light."

The casual attitude toward kissing could be misleading for tourists not in the know. Many Cariocas hold a negative image of Americans, not from the Iraq war, although that doesn`t help, but because of the startling numbers who come thinking they will find a citywide orgy. (Many on Copacabana Beach and at its nocturnal alter ego, the Help disco, end up with prostitutes who pretend not to be prostitutes.)

"It`s another sad thing about tourism in Brazil, that men come here looking for that," said Fabiana Dória, a 24-year-old Carioca I met on Ipanema Beach. "There is so much else here. We are really liberal and we have a lot of human warmth, so people confuse that and take advantage of it."

Hours before prime time at the clubs, many Cariocas begin their evenings in a botequim, one of the charming neighborhood bars with menus that are often surprisingly complete, considering that few patrons seem to order anything but bean soup and appetizers. One of the most popular is Jobi, in the upscale Leblón neighborhood just southwest of Ipanema, where waiters would be well advised to take yoga classes to help them contort their way through the tightly packed tables. Three 20-something women I met in line on an early Friday evening helped me twist my way to a back table for chopp, or draft beer, the usual botequim drink. They ordered a spectacularly gooey pizza calabresa and a bean soup, and the waiter kept track of the chopp consumption with numbered beer coasters he tossed onto the table with each round.

The women, who introduced themselves as Elizabeth, Renata and Tatiana, were soon joined by another friend, Gisela, and we were five. Like everyone else I met, they were generous in welcoming strangers with comical accents and ready to explain the mysteries of life in Rio, like why botequim regulars were so obsessed with deciding which place had the best draft beer even though they mostly sold major national brands like Brahma. ("It`s the way that they pour it," Renata said.)

They began an animated discussion about Carioca men, featuring many complaints - most of which, I regretfully informed them, were true of the sex as a whole. When I left, the coaster tally read 13.

The next stage of the evening is often music, and my pretrip Brazilian advisers had categorized one place as not to be missed: Comuna do Semente, a tiny club that has opened and closed several times over the years and now operates as a nonprofit with a bohemian vibe and an antiglobalization philosophy that bans Coca-Cola.

Semente is in Lapa, a district famous for music clubs and the home of the city`s busiest night life. Semente`s intimate space, next to the raucous street scene but not of it, is practically under the white-arched aqueduct, a sort of 18th-century Brazilian Pont du Gard, that sweeps through and symbolizes the neighborhood.

On the Thursday when I was there, Yamandu Costa performed on the guitar and Gilberto Monteiro on the accordion. I had heard of neither, but it soon became clear that I was alone in this. A hush fell over the room as they started their intense jam session of Brazilian and Argentine rhythms. Acrobatic fans listening from outside hoisted themselves into frames of the open windows and watched from there.

As palpable as the energy of that music scene was, however, I was glad to be in Rio at the right time of year for an entirely different, entirely more populist kind of musical event, the preparation of the city`s samba schools for their Carnaval competition.

In typical Rio style, what was billed as a "rehearsal" at Mangueira, one of the most popular schools, turned out to be part of an ongoing series of exuberant come-one-come-all parties. The school was a lengthy taxi ride into a favela, one of Rio`s poor, crime-plagued neighborhoods that the upper middle class of Baixo Gávea and Rosa Shopping usually avoids at all costs. Affluent Rio was there in force, however, on this Saturday night, paying $7.50 to get in.

Outside, those who couldn`t pay the fee, or didn`t want to, laughed and danced to samba and Brazilian funk, an urban genre heavy on sexual content, on festive and densely packed, if slightly scary, streets. But the main attraction was inside Mangueira, housed in a remarkably pink hall that hinted at what high school gymnasiums would look like if Barbie ruled the world.

The place was packed - or so I thought, mistaking thousands of people milling around it as a crowd. In retrospect, the ability to mill at all should have shown me otherwise. "It`s empty," my friend Carolina said. "Wait until midnight."

A samba band played joyously from a balcony; people danced, drank and caressed with abandon despite the harsh lighting. Several makeshift bars did a brisk business in beers and caipirinhas (and their pinkish strawberry cousins, caipifrutas) as the crowd continued to pour in.

Wrinkled Brazilian women in loose-fitting pink dresses danced merrily; blond tourists bobbed awkwardly. Some didn`t dance at all, but there was no division between wallflowers and samba enthusiasts. Everyone mingled and at least swayed to the music with the same friendly exhilaration.

By midnight, you could barely move. Finally, we could all agree it was crowded.

SETH KUGEL contributes regularly to the City section of The Times