From evidence based on the use of specific receptor subtype antagonists, it has generally been assumed that human uterine tissue contains only type 2 (AT2) angiotensin II (AII) receptor subtype. Using a monoclonal antibody, 6313/G2, directed against a specific sequence in the extracellular domain of the type 1 AII receptor (AT1), in immunocytochemical studies, we show here that AT1 receptor is expressed in human endometrium. In particular, positive staining was seen in the endometrial glandular epithelium, and in the vascular endothelium, while the myometrium and endometrial stroma were negative. The most intense staining was observed during the late proliferative phase and less in the luteal phase. The ligand binding assay, using [125I]-angiotensin II, revealed high concentrations of AII receptors both in the endometrium and in the myometrium. Competition studies using losartan (AT1 specific) and CGP42112B (AT2 specific) showed that both AT1 and AT2 receptor subtypes were present in the endometrium, though only the AT2 receptor subtype was detected in the myometrium. Immunoblotting confirmed that the antibody 6313/G2 detected a single protein with a molecular weight of approximately 60 kDa. These data clearly demonstrate the presence of endometrial AT1 receptors whose expression appears to be under hormonal control.