It is well known that the control of local blood flow in capillaries is controlled by the “precapillary sphincters” (PS) although their nature and the mechanism controlling their function remain controversial. Precapillary pericytes are good candidates for regulating blood flow and perform sphincter function. However, little is known about the mechanisms controlling Ca²⁺ signalling and their contractile activity in situ. We used confocal imaging of in situ rat ureteric microvessels (17 vessels from 15 rats) loaded with the Ca²⁺-sensitive indicator Fluo-4 in order to investigate morphology, Ca²⁺ signalling and mechanical activity of precapillary pericytes. The effects of central (Phenylephrine, PhE) and local (endothelin-1, ET-1) factors as well as caffeine on Ca²⁺ signalling and contraction of precapillary pericytes and their effect on the diameter of the vascular wall of the precapillary branches of the ureteric microvessels (i.d.<10µm) have been investigated. The ureteric microvascular tree consists of 3 generations of microvessels. The last (third) generation of the microvessel has a monolayer of smooth muscle coat in its proximal part followed by a coat of precapillary pericytes running circumferentially around the endothelium in its distal part. Third order branches give off side branches of microvessels which have only a coat of precapillary pericytes (pericytic microvessels). Precapillary pericytes formed an asymmetrical coat with a thick body located on one side of the vessel giving 2-3 fingers like processes which tightly wrapped around the endothelium. Each precapillary pericyte occupied a length of 10.10±0.48 µm of the vessel (n=15). Live staining of nuclei with propidium iodide revealed that pericytic nuclei were curved and were located in the body of the pericyte. Ureteric microvascular pericytes were totally resistant to caffeine (10mM) and PhE (10-100µM) but readily responded to ET-1 (10nM). Precapillary pericytes responded with a single spike-like Ca²⁺ transient resistant to ryanodine and removal of [Ca]_o but which was abolished by SERCa pump inhibitor cyclopiazonic acid (20µM) or inhibitor of IP₃R 2-APB (50µM). The Ca²⁺ transient induced by ET-1 consisted of an initial fast component followed by a slowly decaying sustained component producing local constriction of the microvessel which lasted for 2-3 minutes. Transmitted light imaging revealed that contraction of precapillary pericytes produced complete closure of the microvessel blocking the flow of red blood cells. The data obtained suggest that precapillary pericytes are likely to act as local sphincters controlling capillary blood flow by responding to local factors.