Modifications of uterine blood flow are implicated in many important aspects of reproductive physiology and in several of their pathological disorders. These modifications are hormonally regulated but remain poorly understood, and various complex mechanisms have been proposed. The aim of this study was to investigate the presence and some characteristics of estrogen receptors (ER) and progesterone receptors (PR) in uterine blood vessels. Using monoclonal antibodies and immunocytochemistry we observed the presence of ER and PR in muscle cells (tunica media) of uterine arteries of rabbits and women. ER or PR immunoreactivity was not detected in the endothelium of uterine arteries nor in uterine capillaries or veins. Staining for both receptors was also present in arterial walls from the fallopian tube (isthmus and ampulla) and vagina but not in arteries of nonreproductive tissues (intestinal, renal, hepatic, femoral, and pulmonary arteries, aorta). PR immuno-staining was increased by estrogen in all cell types of the rabbit uterus, but the doses necessary to provoke an intense nuclear staining in uterine arteries were higher than those required for observing strong labeling in glandular, stromal, or myometrial cells. These results suggest that, contrary to many hypotheses previously put forward, sex steroid hormones may regulate uterine blood flow through a direct effect on uterine arterial walls.