Tremendous changes in pressure and flow occur in the pulmonary and systemic circulations after birth, and these hemodynamic changes should markedly affect endothelial cell replication. However, in vivo endothelial replication rates in the neonatal period have not been reported. To label replicating endothelial cells, we administered the thymidine analog bromodeoxyuridine to calves ~1, 4, 7, 10, and 14 days old before they were killed. Because we expected the ratio of replicating to nonreplicating cells to vary with vascular segment, we examined the main pulmonary artery, a large elastic artery, three sizes of intrapulmonary arteries, the aorta, and the carotid artery. In normoxia for arteries < 1,500 µm, ~27% of the endothelial cells were labeled on day 1 but only ~2% on day 14. In the main pulmonary artery, only ~4% of the endothelial cells were labeled on day 1 and ~2% on day 14. In contrast, in the aorta, ~12% of the endothelial cells were labeled on day 1 and ~2% on day 14. In chronically hypoxic animals, only ~14% of the endothelial cells were labeled on day 1 in small lung arteries and ~8% were still labeled on day 14. We conclude that the postnatal circulatory adaptation to extrauterine life includes significant changes in endothelial cell proliferation that vary dramatically with time and vascular location and that these changes are altered in chronic hypoxia.