We hypothesized that disrupted alveolarization and lung vascular growth caused by brief perinatal hypoxia would predispose infant rats to higher risk for developing pulmonary hypertension when reexposed to hypoxia. Pregnant rats were exposed to 11% inspired oxygen fraction (barometric pressure, 410 mmHg; inspired oxygen pressure, 76 mmHg) for 3 days before birth and were maintained in hypoxia for 3 days after birth. Control rats were born and raised in room air. At 2 wk of age, rats from both groups were exposed to hypoxia for 1 wk or kept in room air. We found that brief perinatal hypoxia resulted in a greater increase in right ventricular systolic pressure and higher ratio of right ventricle to left ventricle plus septum weights after reexposure to hypoxia after 2 wk of age. Moreover, perinatal hypoxic rats had decreased radial alveolar counts and reduced pulmonary artery density. We conclude that brief perinatal hypoxia increases the severity of pulmonary hypertension when rats are reexposed to hypoxia. We speculate that disrupted alveolarization and lung vascular growth following brief perinatal hypoxia may increase the risk for severe pulmonary hypertension with exposure to adverse stimuli later in life.