Pulmonary hypertension has been induced in rats by 2 weeks' exposure to hypoxia, equivalent to an altitude of approximately 5500 m, in a hypobaric chamber. The rats were removed from the chamber and allowed to recover for up to 8 weeks at atmospheric pressure. Precise quantitative microscopic techniques after injection of the pulmonary artery have been used to estimate the regression in the pulmonary artery of the structural changes associated with pulmonary hypertension. During recovery the degree of muscularization of the pulmonary arteries decreases by disappearance of muscle cells from the small arteries and a drop in arterial wall thickness of larger vessels. These changes do not seem to reflect pulmonary artery pressure directly, since right ventricular hypertrophy regresses at a faster rate. In hypertensive rats there is a "loss" of small arteries in the alveolar region and little filling of precapillary vessels. On recovery, some of the vessels fill, suggesting that encroachment on the lumen by muscle and endothelial cells has lessened. Even after 8 weeks' recovery, however, some arteries do not return, suggesting they have completely disappeared and that regions are left with relatively little perfusion. This reduction of vascular reserve presents without there being right ventricular hypertrophy.