Oxygen causes perinatal pulmonary dilatation. While fetal pulmonary artery smooth muscle cells (PA SMC) normally respond to an acute increase in oxygen (O₂) tension with a decrease in cytosolic calcium ([Ca²⁺]i), an acute increase in O₂ tension has no net effect on [Ca²⁺]i in PA SMC derived from lambs with chronic intrauterine pulmonary hypertension (PHTN). The present experimental series tests the hypothesis that an acute increase in O₂ tension decreases capacitative calcium entry (CCE) in normal, but not hypertensive, fetal PA SMC. PA SMC were isolated from late gestation fetal lambs after either ligation of the ductus arteriosus (PHTN) or sham (control) operation at 127 days gestation. PA SMC were isolated from the distal PA ( 4th generation) and maintained under hypoxic conditions (~25 torr.) in primary culture. After fura-2 loading, apparent [Ca²⁺]i in PA SMC was determined as the ratio of 340 to 380 nanometer fluorescence intensity. Under both hypoxic and normoxic conditions, cyclopiazonic acid (CPA) increased [Ca²⁺]i more in PHTN than in control PA SMC. CCE was determined in PA SMC under hypoxic and normoxic conditions, after superfusion with zero extracellular Ca²⁺, intracellular store depletion with CPA, followed by superfusion with Ca²⁺-containing solution, in the presence of the voltage-operated calcium channel blockade. CCE was increased in PHTN, compared to control, PA SMC under conditions of both acute and sustained normoxia. Transient receptor potential channel gene expression was greater in control compared to PHTN PA SMC. PHTN may compromise perinatal pulmonary vasodilation, in part, by modulating PA SMC CCE.