The pulmonary vessels of rats treated with fulvine were studied electron microscopically for morphologic signs of contraction of smooth muscle cells. Except for a number of indirect indications of vasoconstriction such as medial smooth muscle cell hypertrophy and excessive crenation of elastic laminae, conspicuous smooth muscle cell excrescences were observed and were interpreted to be a direct result of contraction. The close relationship between contraction and smooth muscle cell excrescences was confirmed by their simultaneous occurrence within 1 minute after administration of histamine to isolated perfused guinea pig lungs. The images of pulmonary vessels of rats with prolonged survival times after fulvine administration suggested a gradual increase in the size of the excrescences with a simultaneous degeneration of their cytoplasmic content, and in some cases their eventual detachment from the main cell body. The latter changes were possibly associated with the widespread vasculitis that often occurred at longer intervals after fulvine application. The smooth muscle cell excrescences in pulmonary veins were generally much more prominent than those in pulmonary arteries. This difference was probably caused by the more rigid structure of the arterial wall whcih prevented the formation of large excrescences. The mechanism of the formation of smooth muscle cell excrescences, their possible general validity as markers of vascular smooth muscle cell contraction, and the implications for the mechanism of action of fulvine are briefly discussed.