Ten adult female Wistar albino rats were subjected to a sub-atmospheric pressure of 380 mm Hg for periods up to 35 days. Five control rats were also studied. The rats exposed to chronic hypoxia developed right-ventricular hypertrophy secondary to hypoxic hypertensive pulmonary vascular disease. Electron microscopy of control rats revealed individual smooth muscle cells immediately beneath the endothelium of pulmonary arterioles. On exposure to chronic hypoxia there was a hyperplasia of immature smooth muscle cells to form a distinct media between external and internal elastic laminae. The external lamina is thick and represents the original single lamina of the normal pulmonary arteriole. The inner lamina is thin and is newly formed internal to the smooth muscle cells. In hypoxic hypertensive pulmonary vascular disease there is no form of intimal proliferation so that the associated pulmonary hypertension is reversible.