A silicone rubber assay is used in conjunction with morphometric measurements to characterize in vitro the contractile properties of retinal pericytes in response to endothelial secreted factors. Factor(s) present in conditioned media derived from pulmonary and retinal microvascular endothelial cells and pulmonary artery endothelial cells promote pericyte contractions. Using a radioimmunoassay significant levels of endothelin immunoreactivity are measured in conditioned media obtained from all three cell lines. Thrombin treatment enhanced endothelin‐like secretions by pulmonary microvascular endothelial cells, but significantly reduced levels of endothelin‐like immunoreactivity secreted by retinal microvascular endothelial cells. Synthetic endothelin and thromboxane A₂ (TxA₂) stimulate pericyte contractions, whereas prostaglandin I₂ (PGI₂) promotes pericyte relaxation. Thrombin and angiotensin II (ang II) have no effect on pericyte contractility. However, using cocultures of pericytes and endothelial cells we observe endothelial‐dependent pericyte contractions in response to thrombin and ang II. Thrombin and ang II stimulate the release of endothelial‐derived contracting factors, with characteristics similar to endothelin. These data suggest microvascular endothelial cell‐pericyte interactions may regulate, at least in part, microvessel contractility.