SummaryFetal and early neonatal left and right ventricular systolic time intervals were assessed by simultaneous records of M-mode echocardiography and electrocardiography. in a longitudinal study in 68 normal subjects. There was no difference between left pre-ejection period (LPEP) and right pre-ejection period (RPEP) before delivery. LPEP remained unchanged from before to within the 30min after delivery but was increased 12h later. RPEP increased from before delivery to within the 30min after delivery. Left ventricular ejection time (LVET) increased almost linearly from before to after delivery. Right ventricular ejection time (RVET) remained unchanged from before to within 30min after delivery. and increased gradually thereafter. LPEP/LVET decreased from before delivery to within 30min after delivery, and changed little subsequently. RPEP/RVET increased from before to within the 30min after delivery. and decreased almost linearly thereafter. As a result, RPEP/RVET was greater than LPEP/LVET within the 30 min after delivery (P0.05). The assessment of cardiac functions with left and right ventricular systolic time intervals is useful to determine the physiological and pathological changes of cardiac dynamics before and after delivery.Tw left ventricular systolic time intervals measured from simultaneous records of the electrocardiogram, phonocardiogram and carotid arterial pulse tracing are useful indicators of left ventricular performance in adults (Weissler et al., 1961, 1968, 1969). An M-mode echocardiogram with an electrocardiogram tracing makes feasible a non invasive evaluation of left and right ventricular function, in both children and adults, and there is an excellent correlation etween systolic time intervals calculated by echocardiography and the conventional methods (Hirschfeld et al., 1975a. b; Stefadouros and With-am. 1975). Circulatory changes in newborn babies and neonates have been studied using the same technique (Riggs et al., 1977; Halliday et al., 1978; Halon et al., 1979; Yoshida et al., 1983).Numerous studies have been done on systolic time intervals in human fetuses, using fetal electrocardiograms and Doppler cardiograms (Goodlin et al., 1972; Wolfson et al., 1977; Robinson et al., 1978: Marata et al., 1978a). With this method, the signals obtained are limited to findings in the left ventricle, as reflected in aortic and mitral valve recordings. With M-mode echocardiograms. the motions of the mitral, tricuspid. aortic and pulmonary valves can be clearly recognised; that is. right- as well as left-sided cardiac valves are visualised (DeVore et al., 1982; Hata et al., 1983c). Therefore. the left and right ventricular systolic time intervals of the fetus in utero can be clearly assessed using simultaneous records of fetal M-mode echocardiograms and electrocardiograms (DeVorr et al., 1981: Hata et al., 1983a).In the present work, we ascertained the fetal circulatory changes during both the antenatal and early neonatal periods in normal human subjects.