Ultrasound Doppler was used to establish time of ductal closure, normal values for blood flow velocity in the pulmonary artery (PA), and time interval between pulmonary valve closure (Pc) and tricuspid valve opening (To) in 37 healthy neonates. Ductal closure had occurred in 23% of the children within 12 h after delivery and in 53% during the next 12 h. No open ductus was found after 30 h of age. Maximal blood flow velocity was 0.90 +/- 0.09 (SD) m/s during the first five days of life and 1.12 +/- 0.17 m/s at the age of 14-30 days. The Pc-To interval is known to reflect systolic PA pressure in adults. The Pc-To interval decreased significantly (p less than 0.01) from an average of 0.059 +/- 0.016 s at 3.5-12 h of age to 0.048 +/- 0.011 at 19-36 h of age and thereafter successively to 0.027 +/- 0.004 s at 20-30 days of age. This value is only slightly higher than that of 0.015-0.020 s for normal adults at comparable heart rates. These data suggest a rather sharp decline of systolic PA pressure during the first day of life and thereafter a slower decline; normal adult values are approached but not reached at 3-4 weeks of age. The Pc-To value seems to be of limited value in the early neonatal period, because even normal neonates have increased values with a large individual variation. After 3-4 weeks of age, an increased value should be taken as an indication of increased systolic PA pressure.