The expression of parathyroid hormone-related protein (PTHrP) in cultured endothelial cells derived from bovine carotid artery was immunocytochemically investigated. The cells showed positive immunoreactivity for PTHrP abundantly in cytoplasm. PTHrP-like immunoreactivity was found in all endothelial cells under investigation. Negative control prepared with non-immune rabbit serum or with the antiserum preabsorbed with PTHrP showed no significant staining, indicating the specificity for immunostaining. Addition of 10% fetal calf serum to serum-deprived endothelial cells did not significantly increase PTHrP-like immunoreactivity. These results suggest that endothelial cells express PTHrP constitutively and PTHrP may act as a new class of endothelium-derived relaxing factor.