To confirm the hypothesis that narrowing of the left pulmonary artery is produced by ductal closure, Doppler velocities in the pulmonary arteries from 20 premature infants with a patent duct were compared to velocities from 20 with a closed duct. In infants with a patent duct, the mean Doppler velocity in the main, right, and left pulmonary arteries were, respectively, 86 cm/sec +/- 5 standard error (SE), 120 +/- 7, and 125 +/- 8. With closed duct, the comparable velocities were 102 cm/sec +/- 7, 129 +/- 8, and 190 +/- 11. The only significant difference was the higher velocity in the left pulmonary artery associated with a closed duct (p = 0.00003). This physiologic origin of left-branch stenosis associated with ductal constriction should be considered when a murmur is detected after administration of indomethacin (in addition to possible persistent ductal flow), and as an explanation for some murmurs in the normal newborn. No anatomic or physiologic support was found for the theories that murmurs in the neonate were due to ductal flow, or to acute angulation of the branch pulmonary arteries.