AFTER DINNER WE GRABBED TITO'S drums and headed out to the jeep. When I say drums, I mean panderetas - tambourinelike instruments that produce a nice fat thwack when slapped by a well-practiced hand.

My friend Tito - on his driver's license his name is H¿ctor Matos - is 100 percent Puerto Rican. Like me, Tito lives on the very Puerto Rican island of Manhattan. He's the lead singer and head panderetero with the group Viento de Agua, whose album I had just produced, and he wasn't about to miss Christmas in Puerto Rico. He knows I'm a rhythm fiend, too, so he invited me down to his parents' home in R¿o Piedras, a nearly 300-year-old town that's now part of the metropolitan sprawl of San Juan. Which is how I wound up spending a week en familia in Puerto Rico during the year's biggest party, and how we wound up in the jeep en route to the rhythmic center of the fiesta.

We detoured to pick up David S¿nchez, the handsome young tenorsax star. David spends most of the year on the run from one jazz festival to another, but like every Puerto Rican who can manage it, he keeps his calendar clear for Christmas. San Juan's perpetual gridlock - the famous tap¿n, or bottleneck - meant it would take at least an hour and a half to get to the heart of the party. We didn't care. David had brought along CDs of Cortijo y Su Combo, so we had Maelo with us.

"Maelo" is the late Ismael Rivera, the maximum idol of Puerto Rican rhythmic music, who in the 1950s was the singer with Rafael Cortijo's famous band. Maelo's voice was raspy, but his phrasing and interpretation were always perfect, his rhythm was ferocious, and he put his whole soul into every note he ever sang. He and Cortijo were pioneers in making hip dance-band records based on traditional Afro-Puerto Rican music. And so we sat in traffic, lost in Cortijo worship.

Whether you love Christmas or hate it, you can't help but love Christmas in Puerto Rico. The Christmas season there is about family and friendship, just like in the States. But it has nothing to do with nostalgia for snow. The only thing that freezes at Christmastime in Puerto Rico is the usual radio playlist; the stations stop spinning their regular records in favor of traditional Puerto Rican end-of-year tunes. It's Christmas music all right, but without the jingle bells and reindeer. There's Spanish-derived music featuring the high-pitched guitarlike instrument called the cuatro, and singers intoning nasal 16th-century-style verses called décimas. Then there's El Gran Combo's "El Arbolito," which is up-tempo salsa. I know the words are about a Christmas tree, but the music says to me: Put on your bathing suit.

Christmas festivities start early in Puerto Rico and go on for a good two months. After Thanksgiving the asaltos begin - traveling nocturnal songsprees, also called parrandas, in which a crew of revelers shows up uninvited at a friend's house. They thump panderetas and sing rhymes demanding food and drink, which their not-very-surprised host has to give them. Then he, too, joins the party as everyone continues along to the next house, and so on until dawn, picking up people as they go. It begins all over again the next night.

The custom of asaltos originated in the days when most people lived on farms, and it doesn't always translate so well to urban apartment buildings. Tito found that out one year when he led a noisy parranda to David's building in Brooklyn. The neighbors were not amused.

On December 26, when people in the cold countries are already taking down their Christmas trees, Puerto Ricans are just hitting their stride. On January 6, Three Kings' Day, the kids get more and bigger presents. Next comes Las Octavitas - eight more days of celebration. By the third weekend in January, everyone's party muscles are all warmed up and it's time for the big blowout, the last hurrah of the Christmas season, the one our jeep was pointed at: the Fiesta de la Calle San Sebasti¿n, a major street festival in the beautifully restored colonial area of Old San Juan.

Tito parked the jeep far from Calle San Sebasti¿n, though it was as close as we could get. From there we proceeded on foot, greeting friends along the way as the rhythm grew louder.

Finally reaching the heart of the action, I think, Now this is a party. Tens of thousands of people are thronging in the street. Puerto Rican flags are everywhere. I don't think I'm imagining it: In the noise, there's a sense of peacefulness and brotherhood.

We step into a joint where a crowd is singing and playing panderetas around a pool table. Tito takes his pandereta out of the case and begins smacking it. Everybody here knows him. They move over to let him in, and the singing gets a little louder.

The style of music they're playing is called plena. It emerged in the southern city of Ponce in the 1920s. Played with panderetas and a scraper called a g¿¿charo, the plena is light and energetic. Traditionally it functioned as a sort of barrio newspaper in song, commemorating events and scandals of the day. Over the years it has become almost a cornerstone of Puerto Rican identity, and though it's not really Christmas music, plena is now part of the glue that binds Puerto Ricans together at this time of the year.

The heart of the plena isn't the professional group; it's the circle of amateurs that forms on the corner. In January, those circles are big. Wherever you look people are singing, and many of them are young; it's considered cool to carry a pandereta.

Soon Tito and David are singing and playing with a group on the sidewalk. I wander off by myself down the hill, where it's darker. In the dim light two blocks away, I spot Andy Monta¿ez, a hero of Puerto Rican music since the 1960s. Barrel-chested like a bomba drum, with an enormous penetrating voice, he's the opposite of a crooner. He doesn't need a microphone. In the dark street he begins to sing with the crowd and clap his hands. He's not getting paid. He's not on a stage. He's not even in the light or on the main drag. No one's saying, "Ladies and gentlemen, please welcome Andy Monta¿ez!" He doesn't have to do this; he's doing it because this is who he is. Everyone is ecstatic.

I wander back up to San Sebasti¿n. I squeeze into the club called Rumba, just in time to see ¿Plen¿alo!, a group from the little town of A¿bonito, take the stage. All of A¿bonito seems to be in the club to cheer them on. Hands go up in the air and stay there. The plena is having its effect.

Some 90 minutes later, Tito's still jamming under a balcony. Three blocks away, someone has dragged a pair of very big speakers into the street. They're playing Viento de Agua's record, and everyone's singing along. Damn, it sounds good.

I lost track of how many nights we did this.

DO YOU HEAR WHAT I HEAR? To catch the Puerto Rican holiday spirit, check out the following CDs. (All titles are available from www.descarga.com.) Truco y Zaperoko: Fusi¿n Caribe¿a (Ryko Latino). Crosses plena with Cuban-style music. Great arrangements. Ismael Rivera y Cortijo: Sus 16 Exitos (AF). Music for all time, featuring Ismael Rivera, aka "Maelo." Viento de Agua: De Puerto Rico al Mundo (Agogo/Qbadisc). Okay, I'm cheating. I produced this. A young Puerto Rican band, with Tito Matos on vocals and panderetas. Willie Col¿n, H¿ctor Lavoe: Asalto Navide¿o, Vol. 2 (Fania). Classic 1970s salsa from New York. Just the thing for Three Kings' Day. El Gran Combo: Nuestra M¿sica (Combo Records). If you don't speak Spanish, you won't recognize this 1985 release as a Christmas album. But it is, complete with "El Arbolito" (The Little Tree). N. S.