Pulmonary vessels of rats were studied electron microscopically after 4 days to 4 weeks of hypoxia and were compared with those of control rats and of "regression" rats that, after a period of hypoxia, were allowed to survive in normal air for up to 4 weeks. Both the hypoxic and the regression rats had medial hypertrophy, not only of arteries and arterioles but also of veins and venules. In contrast to the control and the regression rats, many of the hypertrophic vessels of all types in the hypoxic rats showed signs of constriction, ie, crenation of the wall, indentations of medial smooth muscle cell nuclei, and excrescences of smooth muscle cell cytoplasm, often protruding deeply into the endothelium. These observations permit the following conclusions: a) Vasoconstriction due to hypoxia is not confined to pulmonary arteries and arterioles, as generally assumed, but also occurs in veins and venules. b) Medial hypertrophy may be found in the absence of vasoconstriction; this is especially evident during recovery from hypoxia.