Quantitative autoradiography employing the ETA selective ligand [125I]PD 151242 and the ETB selective ligand [125I]BQ3020 was used to assess the localization of ETA and ETB receptors in human uterus throughout the menstrual cycle. ETA and ETB receptors were present in endometrium and myometrium across the menstrual cycle. In myometrium, neither ETA nor ETB receptor density showed any detectable change across the menstrual cycle. ETA receptors were expressed in stroma throughout the endometrium and showed an increase in density in proliferative endometrium compared with secretory and menstrual endometrium. Endometrial ETB receptors were expressed at low density in the proliferative phase. In the early secretory phase there was an increase in ETB receptor density in the glandular epithelium of the basal region of the endometrium but not in functional endometrium. In the late secretory phase ETB receptor expression was increased in glandular epithelium throughout the endometrium. The highest density of ETB receptors was seen in menstrual endometrium, where they were present in stromal as well as epithelial cells. These results suggest that ovarian steroid hormones may play a role in the control of expression of ETA and ETB receptors in endometrial stromal and epithelial cells respectively.