Doppler echocardiograms were carried out on 51 healthy babies three times during the first 72 hours of life to estimate pulmonary arterial systolic pressure by measuring regurgitant tricuspid jet velocity and applying the Bernoulli equation. Tricuspid regurgitation was detected at some stage in all preterm babies and most of those born at full term. Pulmonary arterial pressure could be measured from peak regurgitant velocity in babies with pansystolic regurgitation. The incidence of pansystolic regurgitation among 34 term babies at 0-12, 13-36, and 32-72 hours of age was 22, 27, and 19%, and in 17 preterm babies (within the same age groups) was 53, 50, and 31%, respectively. Estimates of pulmonary artery pressure in the term babies were in accord with known catheter values. Pressure fell rapidly during the first day in all 51 babies. The ratio of pulmonary:systemic arterial pressure was comparable between the two groups throughout. Ductal flow patterns mirrored the fall in this ratio with age--bidirectional flow was associated with a ratio of between 0.88:1 and 1.22:1 and high velocity left to right flow with a ratio of between 0.49:1 and 0.66:1. Both these techniques are noninvasive ways of assessing neonatal pulmonary arterial pressure.