In two experiments to study the effects of chronic hypoxia on the pulmonary vasculature, female weanling rats and male adult rats were kept in a hypobaric chamber for 5 weeks at a pressure of 380 mm Hg. In the first experiment, the female weanling rats developed pulmonary hypertension, right ventricular hypertrophy and an increase in thickness of the smaller pulmonary arteries and arterioles. In the second experiment the male adult rats showed right ventricular hypertrophy and an increase in thickness of the pulmonary trunk, but lacked changes in the pulmonary arteries or arterioles. In both experiments the observed changes reversed after recovery for 5 weeks at normal atmospheric pressure. These experiments demonstrate that chronic hypoxia rapidly produces a reversible pulmonary hypertension which does not necessarily involve structural alterations in the pulmonary vasculature, and suggest that pulmonary vasoreactivity may be influenced by sex and age.