The fine structure of blood vessels with a diameter of less than 50 micrometers, i.e. small arterioles, capillaries and postcapillary venules were studied. The number of pinocytotic vesicles was greater in capillary endothelium than in arterioles and venules. There was regional variation in vesicle frequency in capillary endothelial cells (arteriolar and venule cells not studied). Fenestrations and pinocytotic vesicles were closely associated in some capillary-endothelial membrane areas and segmental differentiation of tight junctions (zonula occludens) in the vascular endothelium. The arteriolar tight junctions were classified as relatively tight, capillary junctions as leaky and venule junctions as very leaky. No nexuses (gap junctions) were found in the endothelium. Only four nexuses were found in the arteriolar smooth-muscle cells. Perivascular fibrocytes were frequently interconnected with nexuses. Nerve fibres in the vicinity of blood vessels contained granular and clear vesicles. The narrowest gap between vesicle-containing nerves and smooth-muscle cells was 0.3 micrometer. No specialized contacts were found between nerve fibres and vascular cells. Solitary cilia with a 9 + 0 pattern were occasional in capillary endothelial cells fibroblasts and odontoblasts. Only one arterio-venous anastomosis was found.