Ectopic ventricular activity in acute myocardial infarction is considered to be benign if it is slow and regular (accelerated idioventricular rhythm), but ominous when rapid (ventricular tachycardia). However, it has been observed in an increasing number of reports that these two types may coexist in the same patient, altering thereby the clinical significance of both. In the present study electrocardiograms were analyzed of 55 patients hospitalized for acute myocardial infarction, in whom idioventricular rhythm occurred. It was found that three major types of ventricular rhythms could be identified: a regular-stable rhythm, an irregular-unstable one, and a third variant which was a combination of these two types. The stable ventricular rhythm was self limited and harmless. The unstable and combined types which were characterized by random coupling times and varying interbeat intervals, were frequently associated with re-entrant beats and fast ventricular rates and therefore a potentially ominous prognosis. It is suggested that the Ca<sup>++</sup> dependent slow diastolic depolarization may be the mechanism responsible for the unstable ventricular rhythm, and the reasons for this assumption are discussed. A therapeutic approach based on the above considerations is described.