Little is understood of the mechanisms involved in reducing pulmonary arterial wall thickness on recovery from pulmonary hypertension and the present study sought to clarify the events that occur. Piglets were exposed to hypobaric hypoxia for 3 days, either from birth or from 3 days of age, and others were exposed for 11 days starting at 3 days. All recovered in room air for up to 6 days. Using light and electron microscopy, the pulmonary artery wall thickness, the relative contribution of smooth muscle and matrix, smooth muscle cell replication, and apoptosis were assessed after hypoxic exposure and during recovery from hypoxic exposure. In elastic arteries, after 6 days' recovery in room air, a reduction in wall thickness to normal was associated with a similar reduction in proportional area of smooth muscle cells and matrix (p < 0.05), increased apoptosis (p < 0.05), and an abnormally low replication rate (p < 0.05). In peripheral muscular arteries, an increase in external diameter, and wall thinning on recovery, was achieved by smooth muscle cell remodelling and a reduction in cell replication (p < 0.05). Apoptosis did not contribute. Thus, different mechanisms are involved in recovery from hypoxia-induced pulmonary hypertension in elastic and muscular pulmonary arteries. Recovery is slower in animals exposed from birth rather than from 3 days of age.