To evaluate the effects of altered preload on the Doppler flow pattern of right ventricular inflow in the first day of life, serial Doppler echocardiography of the pulmonary artery (PA) and tricuspid valve was performed in 16 normal neonates at 2, 12, and 24 hr. A computer-interfaced digitizer pad was utilized to measure the followings: PA flow velocity-time integral, total transtricuspid flow velocity-time integral, flow velocity-time integral of early diastolic filling (E area), and flow velocity-time integral of atrial contraction (A area). The PA flow velocity-time integral, total transtricuspid flow velocity-time integral, E area, A area, peak E, and peak A were increased significantly by 24 hr compared with the values at 2 hr of age. However, the peak E/A, E/A area and peak E/total transtricuspid flow velocity-time integral did not show significant changes from 2 to 24 hr. The ductus arteriosus size was inversely correlated with the peak E (r = -0.46, p < 0.05), E area (r = -0.37, P < 0.05), and A area (r = -0.34, p < 0.05). Therefore, the increase in right ventricular preload at 24 hr is considered to be due to the cessation of the left-to-right shunt through the ductus arteriosus. These results suggest that the pattern of early and late diastolic filling of the right ventricle were dependent on the increase in preload. We can therefore conclude that the redistribution of transtricuspid flow induced by the preload condition should be taken into account to interpret the transtricuspid velocity pattern in terms of diastolic function.