Recent physiological studies have indicated the significant role of pulmonary veins in the total resistance of pulmonary vasculature. The structure of pulmonary veins in the rat was reinvestigated to clarify the different venous segments and their ultrastructure with regard to the musculature including cardiac muscles and smooth muscles with light and electron microscopy. The cardiac muscles were located in the axial and the primary branches of the pulmonary veins within a certain distance limit from the hilum (CM segment) and not in the peripheral region (non-CM segment). The smooth muscles were found indifferent to the presence of cardiac muscles as a continuous layer in segments larger than 180 microm (continuous SM segment) or as a discontinuous layer of circular smooth muscle cells in segments between 50 and 180 microm (partial SM segment). The smooth muscle layer was extremely thin in the CM segments, whereas it became conspicuously thick in the non-CM segment with an irregularly undulating luminal outline, especially in the partial SM segments. There were two elastic laminae in the CM segments: a conspicuous one on the interstitial side of the smooth muscles, and a weaker one between the endothelium and smooth muscles. In the non-CM segment, one elastic lamina was found on the interstitial side of the smooth muscles. Considering the limited range of contraction of cardiac muscles and the thinness of smooth muscle cells in the CM segments, it was concluded that vasoconstriction in the pulmonary veins is executed by smooth muscle cells in the non-CM segments thicker than 50 microm.