87 24-hour electrocardiographic monitoring sessions were conducted in 71 ambulatory patients (pts) in the early post-hospital phase after acute myocardial infarction. 29 (41%) had anterior, 42 (59%) inferoposterior myocardial infarction, 44 (62%) had angina or asymptomatic ischemia, 7 (10%) were in cardiac failure (II-III class NYHA). The occurrence of ventricular ectopic activity (VEA) during sleep hours was compared to the awake state; VEA during sexual and other activities of the awake state was studied. Excluding pts free of VEA during 24-hour monitoring the number of ventricular mature beats was lowered in 71% of sessions and augmented in 12% during sleep hours (p less than 0.001). Maximal grade of VEA was observed during wakefulness in 62% of session and in 9% during sleep (p less than 0.001); in 9% there was no difference, in 20% no VEA occurred. Total VEA, bi-tri-quadrigeminism and repetitive extrasystoles were significantly reduced during sleep: p less than 0.001, p less than 0.05 and p less than 0.01 respectively. This pattern was independent from site of infarction, presence of ischemia, cardiac failure (II-III class NYHA). A discrete correlation was observed between total VEA and heart rate; a poor correlation was seen between total VEA, multiform and repetitive extrasystoles. During wakefulness VEA was mainly related to physical exertion and emotional stress. Sleep and relax periods of the day showed VEA similar to sleeping hours of the night. Sexual activity did not significantly elicit more VEA than other activities of the awake state. Repetitive forms, observed in 8% of the population, were probably related to the rapid increase of sympathetic tone. The absence of repetitive VEA, potentially dangerous, during sleep hours (except the first) not due to antiarrhythmic drugs suggests to increase the dosage of these medications during the awake state in the majority of these pts.