We investigated the effects of prostaglandins (PG) E1, I2 and D2 on the pulmonary vascular bed of newborn rats subjected to chronic hypoxia. Forty seven newborn rats were exposed to chronic hypoxia (10% oxygen) for 23 days and divided into five groups which received PGE1 (N = 10), PGI2 (N = 9), PGD2 (N = 11), placebo (N = 9) or served as controls (N = 8). The PG's were administered by implanting slow release subcutaneous pellets delivering an average daily dose of 1 microgram. Thirteen newborn rats in room air received either placebo or served as controls. All rats were killed after 23 days and heart-lung preparations were perfusion fixed with glutaraldehyde. Appropriate blocks were processed for quantitative morphometry of the pulmonary parenchyma, arteries and arterioles. For analysis the pulmonary vessels were grouped by external diameter, i.e. 30-50 mu and 50-100 mu. External diameter/lumen diameter, an index of medial smooth muscle mass, was 1.39 +/- 0.02 in room air controls, this index of medial muscle mass was significantly increased (p less than 0.01) to 1.53 +/- 0.03 in hypoxic controls and the PGD2 and PGI2 groups. However, the PGE1 group did not have medial hypertrophy evidenced by a medial muscle mass index of 1.34 +/- 0.04, similar to room air controls. These findings suggest that PGE1 may prevent the development of medial hypertrophy that occurs in chronic hypoxia.