Pericytes are contractile cells found on the abluminal surface of capillaries. In the CNS they bidirectionally regulate capillary diameter (Peppiatt et al., 2006) and constrict irreversibly in ischaemia resulting in a prolonged blood flow decrease even after a thrombus in a feeding artery has been cleared (Yesmisci et al., 2009). They also regulate the permeability of the blood-brain barrier and play a role in angiogenesis and formation of the glial scar after injury. Pericytes may, therefore, play a significant role in CNS health and disease. The heart, like the brain, can suffer a “no-reflow” phenomenon after ischaemia (Krug et al., 1966), raising the possibility of ischaemia-evoked capillary level constriction. We investigated the density and anatomical characteristics of myocardial pericytes in the rat to determine whether these suggested a potential role in controlling capillary diameter and blood flow. P12 (n=6) and P30 (n=5) Sprague-Dawley rats were sacrificed by Schedule 1 methods, prior to removal and immersion fixation of their hearts in paraformaldehyde. Transverse ventricular sections (100μm thick) were prepared for immunohistochemistry using a vibratome. Ventricular pericytes were labelled for NG2 and/or PDGFRβ, and capillaries were labelled with FITC-conjugated isolectin B4. Laser scanning confocal microscopy was used to gather image stacks for pericyte quantification. Data were analysed offline using ImageJ software. In the left and right ventricle of P12 rats, pericytes were present at a density of 4.4×107/cm3 and 4.8×107/cm3 respectively (not significantly different, p=0.17, t-test). In P30 rat left ventricle, pericytes were on average separated by 35±3µm along capillaries and were present at a density of 2.4×107/cm3. The capillary density at P30 was 84,000/cm2. Pericytes extend NG2-positive processes along and around coronary capillaries, providing a potential anatomical substrate for control of capillary diameter. Some of these processes make repeated small (~1µm) diameter pads of contact onto the capillaries along their length, the function of which is unknown. Pericytes are as abundant as cardiac myocytes in the ventricular tissue of young rats, and are present at a similar density in both ventricles. The high density of pericytes in the heart suggests a potential role in regulating myocardial blood flow or capillary permeability.