The sarcomere dynamics of enzymatically isolated cardiac myocytes obtained from guinea pig ventricles was studied by the laser diffraction method (He-Ne laser, 3 mW). Unattached cells were stimulated electrically with a frequency of 0.5 s-1 thus producing stable contraction relaxation cycles for more than five minutes. The extent of shortening varied from cell to cell between 0.05 micron and nearly 0.25 micron/src (src = sarcomere) in the steady state. Starting with a rested state contraction repetitive stimulation resulted in a positive staircase and a nearly threefold extent of shortening after reaching the steady state. Synchronous contractions often consisted of two components that were changed markedly during application of ultrasound (5 W cm-2, 1 MHz). The late component disappeared completely while the first one increased continuously. Differences in extent and velocity of sarcomere shortening are probably due to different contributions of the sarcoplasmic reticulum to the total amount of calcium that controls the myofilament activation.