The effects of digoxin on systolic and diastolic time intervals were studied in 25 children and infants the majority of whom had congenital heart disease by M mode echocardiography. The recordings were performed before and after the administration of digoxin. Serum digoxin levels were measured to confirm therapeutic dosage. After digoxin, the right and left ventricular pre-ejection periods, the duration of the corrected electromechanical systole and the Weissler indices decreased, and the isovolumic relaxation periods increased. The ventricular ejection times were unchanged except for the corrected right ventricular ejection time which was only slightly decreased. Our results concerning left ventricular systolic time intervals are in agreement with other studies in children. As no other studies of the effects of digoxin on the right ventricular systolic time intervals, or of the right and left isovolumic relaxation time are available, confirmatory studies are required. The decrease in the right and left pre-ejection periods, electromechanical systole and the Weissler indices, is interpreted as being related to the positive inotropic effect of digoxin whilst the increase in isovolumic relaxation reflects only a decrease in preload. This study allows a better understanding of the effects of digoxin on the different phases of the cardiac cycle and a better appreciation of its action potential.