It has long been recognized that the control of intermittent blood flow in capillaries is controlled by the “precapillary sphincters”. However, until now it has not been clear what type of cells performs this important physiological function. Suggestions are: smooth muscle cells (SMC) originating from precapillary arterioles (PA) (Rhodin 1967) encircling capillary orifices, precapillary pericytes expressing contractile proteins (Hirshi et al, 1996, Sims 1986) and even specialised contractile endothelial cells (Ragan et al, 1988). However, to the best of our knowledge no study has investigated the properties of these cells in situ. In the present study we used high-resolution X-Y-Z and fast temporal confocal imaging of in situ ureteric microvessels loaded with the Ca²⁺-sensitive indicator Fluo-4 (Burdyga et al, 2003) in order to investigate morphology, Ca²⁺ signalling and mechanical activity of SMC and precapillary pericytes comprising rat ureteric precapillary sphincter (PS) (inner diameter, i.d.<10µm). The effects of central (phenylephrine, PhE) and local (endohelin-1, ET-1) factors as well as caffeine – a powerful vasoconstrictor on PA (i.d.=15-25µm) and PS (i.d.<10µm) have been investigated. We have defined two types of PS: type 1 (length >40µm) in which endothelium was encircled by both SMC and precapillary pericytes; and type 2 (length <40µm) in which endothelium was encircled by a group of precapillary pericytes only. SMC made three full turns around the endothelium and occupied a length of 9.81±0.21µm (n=12). Precapillary pericytes formed an asymmetrical coat with thick massive body located on one side of the vessel giving several finger like processes wrapping around endothelium. Each precapillary pericyte occupied a length of 10.10±0.48µm (n=15). SMC of PA responded with Ca²⁺ oscillations and vasomotion when stimulated by either PhE (10µM) or ET-1 (10nM). Also in these myocytes, caffeine at low concentrations (0.5-1mM) enhanced or initiated Ca²⁺ sparks and at higher concentrations (2-5mM) produced Ca²⁺ waves and vasoconstriction. In marked contrast, SMC of PS were immune to caffeine (1-10mM) and PhE (10-100µM) but readily responded to ET-1 with Ca²⁺ oscillations and vasoconstriction. Precapillary pericytes were also insensitive to either caffeine (10mM) or PhE (10-100µM) but readily responded to ET-1 (10nM) with a non-oscillatory Ca²⁺ transient which consisted of an initial spike followed by sustained component associated with strong vasoconstriction. The data suggest that precapillary pericytes are more likely to act as local sphincters controlling capillary blood flow responding to local factors.