The early changes in pulmonary artery structure and the timing and development of right ventricular hypertrophy were examined in rats after ingestion of Crotalaria spectabilis; the arteries were distended with a Micropaque--gelatin mixture. The earliest change, detected at Day 3, is the appearance of new muscle in normally nonmuscular arteries. Increased medial wall thickness of the normally muscular arteries is apparent from Day 7, reaching significance in the smaller arteries from Day 10 and in the larger arteries from Day 14. A reduction in number of small, filled peripheral arteries is apparent at Day 14 and is significant at Day 21 as is the increase in the ratio of alveoli to arteries per unit area of section. Right ventricular hypertrophy is detected and significant at Day 21 and, like all the arterial changes, continues to increase to Day 35. Hypoxia produces similar changes to those described above, although without the inflammation seen after Crotalaria ingestion, but the time table is different. New muscle appears in both models at the same time, but the other changes appear more slowly after ingestion of Crotalaria spectabilis.