American ears aren't used to it yet, but rhythm and polyrhythms can yield just as much variety and complexity as melody and harmony. Nigeria's King Sunny Ade & the African Beats and Jamaica's Black Uhuru with Sly & Robbie demonstrated the rich possibilities of the beat at Constitution Hall last night.

Ade's 18-member troupe included eight percussionists, each of whom supplied his own rhythmic pulse pattern. All these patterns synchronized flawlessly into rolling waves of rhythm that rose on common accents, fell into scattered polyrhythms and rose again. Ade and his three fellow guitarists lent a melodic flavor and electronic tingle to the rhythms. Ade and his four fellow singers harmonized on the sweet, cool tunes with Yoruba lyrics and then stepped back for exuberant, leg-shivering dances as the talking drums and pedal steel guitarist took over.

The key to Black Uhuru's opening set was drummer Sly Dunbar who played a rock-steady backbeat on his trap drums and a countering five-beat phrase on his electric drums; he elaborated the songs from there. Dunbar's partner, Robbie Shakespeare, played the melody at an agitated, staggered pace on the bass. The band added splashes of synthesizer, electric guitar and wood blocks to the dense, aggressive rhythm, which was quite different from the lazy lope of much reggae.

Lead singer Michael Rose of Black Uhuru stretched drawn-out, patient vocals over this busy beat. Leaving plenty of room for the instrumentalists, Rose sang Black Uhuru hits like "Guess Who's Coming to Dinner" and "Shine Eye Gal" as well as Steve Van Zandt's "Solidarity."