It has been shown that there are pressure gradients between the main pulmonary artery (MPA) and its two branches in infants undergoing catheterization. This study investigated the blood flow velocities and pressure gradients in the right and left pulmonary arteries (RPA and LPA, respectively) in normal neonates. The MPA and its two branches were examined echocardigraphically in 114 term consecutive healthy neonates aged 1-6 days. The pressure gradients between the MPA and RPA or LPA were calculated. Thirty neonates with pressure gradients above 2.5 mmHg were followed by 3-6 months. The peak velocities in the RPA and LPA (1.16 +/- 0.19 and 1.01 +/- 0.18 m/s) were significantly higher than that in the MPA (0.84 +/- 13 m/s) (both p < 0.001), with that in the RPA slightly higher than in the LPA (p < 0.001). There was an estimated pressure gradient of 2.5-8.3 mmHg between the MPA and RPA in 43% and of 2.5-6.6 mmHg between the MPA and LPA in 16.7% of all neonates. The gradients disappeared within 3-6 months in 12 (40%) of the 30 neonates with an initial gradient above 2.5 mmHg. The differences in blood flow velocities or pressure gradients in the RPA or LPA were probably attributable to the variations in pulmonary arterial pressure, cardiac output, age, and birth weight and can be considered physiologically characteristic in neonates.