The characteristics of depolarization-induced automaticity in myocardium and the mechanisms presumably responsible for their occurrence and maintenance were discussed in relation to ionic channels and ion transport systems of the cell membrane. The initiating mechanisms of automaticity were subdivided into oscillatory potentials of increasing amplitude, spontaneous slow depolarization and sustained depolarization of the membrane, and various possible causes of these potential changes were described. As for the mechanisms responsible for the maintenance of automaticity, phase 4 depolarization, after depolarization and triggered activity were considered with special reference given to the excitability recovery characteristics during the diastolic period. Thus, various factors were responsible for modifying the automaticity. Also, intracellular Ca fluctuations which induce oscillatory potentials, afterdepolarizations, or even supernormal periods of excitability appear to play a cardinal role. Clarification of the intracellular Ca metabolism in relation to internal membrane systems such as the sarcoplasmic reticulum and mitochondria should yield pertinent information resulting in a better understanding of these events.