Idiopathic persistent pulmonary hypertension of the newborn infant (PPHN) is characterized by intrauterine structural remodeling of the pulmonary arterial bed, consisting of precocious development of muscle in intraacinar arteries, proliferation of adventitial connective tissue, and sometimes medial hypertrophy of preacinar arteries. To evaluate whether gestational hypoxemia causes these changes, we studied pulmonary arterial structure in two groups of newborn rats: one control, the other exposed to hypoxemia produced by maternal hypoxia during the second half of gestation. Morphometric analysis of the pulmonary arterial bed was performed after barium injection into the pulmonary arteries and formol saline expansion of the air spaces. Birth weight was similar in each group. Hematocrit was elevated in the hypoxemia group (51% +/- 1.0% vs 46% +/- 0.8%, P less than 0.005). The structure of preacinar and intraacinar arteries was similar and normal in both groups. Chronic fetal hypoxemia in the rat does not produce the pulmonary arterial structural changes identified in fatal cases of PPHN in human infants.