Progesterone receptors (PgR) were identified in 31 of 50 specimens of human (men and women) thoracic ascending aorta, internal carotid, coronary artery, and left atrial appendage. This was accomplished with a peroxidase-antiperoxidase immunocytochemical assay employing a highly specific monoclonal antibody to primate PgR. In the aorta, specific staining was seen in the nuclei of smooth muscle cells and endothelium of intima, media, and adventitia. In the myocardium, staining was localized to the nuclei of the myocardial fibers. In internal carotid and coronary arteries, PgR was localized to endothelial nuclei of intima, and in vascular channels within the atherosclerotic plaques. PgR was also visible in the smooth muscle cell nuclei of uninvolved media and intima and at the plaque periphery. In contrast, receptor was not identified in vessels of the human uterus, breast, prostate, kidney, or gastrointestinal tract. These findings suggest that the heart and great vessels are target organs for steroid hormones.