Systolic time intervals (STIs) were obtained from simultaneous recordings of electrocardiogram, phonocardiogram and echograms of aortic and pulmonary valves in normal school age children. Pre-ejection time, ejection time and the ratio of the former to the latter were analyzed on both sides of the heart. Analysis was performed on 171 cases on the left and 144 cases on the right side of the heart. Because of the technical difficulty in detecting echograms of pulmonary valve closure, the onset of the second heart sound was employed in place as the end of right ventricular ejection time. This might bring about errors in the values of right ventricular ejection time, and, therefore, the ratio of right pre-ejection period. The error ratios of both of these indices were estimated to be about ten per cent. Ejection time was closely correlated with the cardiac cycle, suggesting that it is affected by the heart rate. The other indices appeared to be independent of the heart rate. There was virtually no difference attributable to age or sex. Mean and standard deviation of each index was (m±SD) : LPEP : 0.07±0.01 sec, RPEP : 0.06±0.01 sec, LVET : 0.27±0.02 sec, RVET : 0.28±0.03 sec, LPEP/LVET : 0.25±0.04,RPEP/RVET : 0.23±0.04. The upper limits of the latter two were 0.35 and 0.30,respectively.