In 55 Sprague-Dawley rats (mean wt, 277 +/- 6.2 g) exposed to hypobaric hypoxia (air at 380 mmHg), and 23 weight-matched controls kept in room air, pulmonary and systemic artery pressures were measured daily for 2 wk via indwelling catheters. After each day of exposure, 1 or 2 hypoxic rats, to a total of 20, and 5 control rats were killed during the experiment. In these rats, the pulmonary arterial tree was injected post mortem with barium-gelatin and inflated with formaldehyde solution, and three structural features were quantified microscopically: 1) abnormal extension of muscle into peripheral arteries where it is not normally present (EMPA); 2) increased wall thickness of the normally muscular arteries, expressed as a percentage of external diameter (%WT); and 3) reduction in artery number expressed as an increase in the ratio of alveoli to arteries (A/a). Mean pulmonary artery pressure (Ppa) rose significantly after day 3 of hypoxic exposure (P less than 0.05) and had doubled by day 14; the mean systemic artery pressure (Psa) of hypoxic rats and Ppa and Psa of control rats were unchanged. The level of Ppa correlated with the degree of structural changes; for EMPA, r = 0.84; for %WT, r = 0.64; and for A/a, r = 0.73 (P less than 0.001 in all.