Myocardial cell groups, mechanically disaggregated from the rat heart, beat spontaneously in a medium which resembles intracellular fluid. The cells, initially functioning in an uncoordinated way, i.e. each cell in its own rhythm, gradually become coordinated as a function of time. After 90 min of incubation at 15 degrees C all the cell groups were functioning in a coordinated way. The initiation of coordination was temperature-dependent. Lowering the temperature from 15 degrees C to 10 degrees C significantly delays the coordination, only 75% of the cell groups being coordinated after 90 min. The contraction wave in coordinated cell groups was very slow; 90 +/- 7 micrometers/s at 20 degrees C, 100 +/- 6 micrometers/s at 30 degrees C and 100 +/- 7 micrometers/s at 35 degrees C. The velocity of contraction waves was not significantly temperature-dependent, suggesting a neonenzymatical type of propagation. It also seems too slow to be an electrical event. Apparently the contraction waves are propagated by diffusion of Ca2+. The addition of caffeine to the medium hastened the coordination of contractions, but Na-azide strongly inhibited it. The results indicate that the sarcoplasmic reticulum functions in the coordination of contractions, probably by releasing Ca2+ for the activation of the myofilaments.