We performed echocardiography on neonates sequentially and investigated the changes in hemodynamics during the neonatal period. Twenty newborn infants (male : ten, female : ten, gestational age : 34-41 weeks, birth weight : 1540-4100g) were examined. The first examination was performed within one day after birth, and the second examination two to six days thereafter. Six indices of hemodynamics were analyzed on both sides of the heart; the ratio of pre-ejection period to the ejection time (LPEP/LVET, RPEP/RVET), end diastolic ventricular dimension (LVDd, RVDd), and shortening fraction of ventricular dimension (SFLVD, SFRVD). Gestational age or heart rate did not affect these indices, while birth weight closely correlated with LVDd and RVDd (r=0.44 and 0.66,respec-tively). The mean±standard deviation of the indices at the first and the second examinations were : cardiac cycle : 0.48±0.06 sec and 0.47±0.08 sec, LPEP/LVET : 0.34±0.07 and 0.28±0.06,RPEP/RVET : 0.41±0.11 and 0.27±0.06,LVDd : 1.76±0.20cm and 1.79±0.21cm, RVDd : 1.50±0.20cm and 1.61±0.22cm, SFLVD : 0.32±0.06 and 0.35±0.06,SFRVD : 0.26±0.08 and 0.29±0.07. In the series of examinations, LPEP/LVET and RPEP/RVET decreased and LVDd and RVDd increased, suggesting that the reduction in the after-load on both ventricles, the improvement of left ventricular contractility, and the increase in the preload of the ventricles may take place during the first week after birth.