The three-dimensional relationships between smooth muscle cells and endothelial cells and between pericytes and endothelial cells in four segments of the microcirculation were analyzed by computer reconstructions from serial electron micrographs. In elastic-containing arterioles, the smooth muscle cells formed an inner longitudinal layer above and parallel to the elastica and an outer spiral layer. In the terminal arterioles the two layers of smooth muscle cells and elastica were replaced by a single smooth muscle cell that completely encircled the endothelial cell tube. The pericytes in the post-capillary venules completely encircled and gripped the endothelium through multiple contact points from their lateral processes. In the large venules the pericytes only partially encircled the endothelial cell tube and were more randomly placed.