The influence of endothelial cells on the growth of vascular pericytes was investigated using human umbilical endothelial cell and bovine retinal pericyte co-culture systems. When pericytes were cultured with endothelial cells, the number of viable pericytes increased significantly, regardless of the presence or absence of physical contact between the two cell types, indicating that endothelial cells can induce proliferation of pericytes through certain secreted factor(s). Evidence that endothelin 1 mediates this action was obtained: endothelin 1 added to the medium was able to substitute for the feeder layer of endothelial cells, and an antiserum against endothelin 1 could neutralize the ability of endothelial cells to promote pericyte growth. RNA blot analysis revealed that pericytes express mRNA for type-A endothelin receptor, a high affinity receptor of endothelin 1.