Fine structural changes of cardiocytes in the chick embryo before and after the onset of rhythmical contraction (Hamburger-Hamilton stages 8 to 17) were studied by light and electron microscopy. The earliest heartbeat was recognized at stage 10 by means of dissecting microscopy and the incidence of beating hearts increased as stage proceeded. In precontracting hearts, cardiocytes contained a small number of actin-like filaments, but thick filaments and primitive myofibrils were not discernible. At stage10, when some hearts are considered to commence rhythmical beating, myosin filaments and primitive myofibrils appeared in a few cardiocytes. The spatial orderly arrangement of actin and myosin filaments appears to play an important role for the onset of the first heartbeat. Desmosomes and intermediate junctions were well developed even in prebeating hearts. Elements of sarcoplasmic reticulum were rarely observed at postheartbeat stage 12 and remained infrequent in later stages of development. Thus these vesicles are unlikely to correlate closely with contraction in young embryonic hearts. It is clearly demonstrated that hearts are able to contract in an immature condition when not all but some cardiocytes contain only a few developing myofibrils.