Pulmonary arterial pressure (PAP) was increased obviously in rats after 3 days of normobaric hypoxic exposure and reached a maximum at 14 days of hypoxia. It remained at the same level during prolonged hypoxic exposure of up to 21 days. Right ventricular weight (RV/LV+S) and hydroxyproline (HP) content in the pulmonary artery began to increase at day 7. HP content had increased much faster than the relative rate of increase of PAP after 14 days, but HP content in the thoracic aorta showed no change. The relative proportion of type I to III collagen increased significantly, and compliance of the pulmonary vessels obviously decreased. All parameters returned to the normal range within 14 days after recovery from hypoxia, except for HP content as expressed per vessel. 764-3 treatment obviously attenuated most of the changes caused by hypoxia, though it had no effect on compliance of the pulmonary vessels. It is suggested that collagen, especially type I collagen, accumulation may play an important role in maintaining pulmonary hypertension. 764-3 has certain protective effects and may be useful in the treatment of early chronic obstructive pulmonary disease.