The pericytes of capillaries are interesting cells which resemble the smooth muscle cells of larger vessels in some aspects of their morphology and behavior. In this report, their relationship to the underlying endothelium has been investigated in some detail. Using indirect, fluorescent immunocytochemical techniques on fresh and fixed tissues, it was found that fibronectin (an adhesive protein in many tissue culture systems) is concentrated in spots along vessels and is only faintly visible in the basement membranes of exhaustively perfused preparations. By electron microscopy, using a peroxidase immunocytochemical marker, these concentrations of fibronectin were seen to be localized to the pericyte-endothelial interstitia. Examination by TEM using a new fixation procedure demonstrated the organization of microfilaments and dense plaques along the pericyte membrane with fibrous and basement membrane-like material within this interstitial space. The arrangements of these elements suggest a mechanical linkage between the two cells. Such a linkage would allow contractions or relaxation of the pericyte to affect vessel diameter.