Thickening of peripheral pulmonary arteries (PA) in the pulmonary hypertensive neonate has been well described morphologically, but less is known regarding the role of cell proliferation in either the normal or hypertensive neonatal PA. Thus we studied DNA synthetic indices in the tunica media and tunica adventitia of four different sizes/generations of PA in normoxic calves (n = 15) and calves exposed to hypobaric hypoxia (n = 15) during the first 14 days of life. DNA synthetic indices were determined by incorporation of the thymidine analogue bromodeoxyuridine (BrdU). Hemodynamic studies confirmed a steady decline in PA pressure in normal neonatal calves during the first 2 wk of life and progressive pulmonary hypertension in the hypoxic group. Lungs were perfusion-fixed and pulmonary arteries were evaluated for BrdU incorporation by immunohistochemistry. DNA synthetic indices (BrdU-labeled cells/1,000 cells) in the tunica media from normoxic calves were highest between 4 and 7 days postpartum and decreased to their lowest levels by day 14. The highest indices were observed in smaller generations of PA in the normoxic newborn. Adventitial cells exhibited the same general pattern of BrdU incorporation except that the postpartum peak occurred earlier, at 1 to 4 days. Exposure to hypoxia significantly increased (P = 0.001) DNA synthetic indices in both the tunica media and adventitia. The highest DNA synthetic indices were observed in smaller-generation vessels. These findings indicate that the fraction of cells traversing the S phase (i.e., actively replicating in the cell cycle) in the normal neonatal pulmonary vasculature during transition are initially high compared to reported rates in hilar PA from adult rats, but then decrease by 14 days after birth. Further, exposure to hypoxia during transition dramatically increases and prolongs pulmonary vascular cell proliferation. We conclude that structural remodeling in the hypertensive neonatal PA is due partly to increased cell proliferation in the tunica media and adventitia.