Uterine arterial blood flow dose-response curves to systemic infusions of either estrone (E1) or estradiol-17alpha (E2alpha) were determined in ten oophorectomized, nonpregnant ewes. Maximum uterine blood flow occurred with 5 microgram per kilogram of E1 and greater than or equal to 20 microgram per kilogram of E2alpha. Total uterine blood flow measured with isotope-labeled microspheres rose more than tenfold following 30 microgram per kilogram of either estrogen, as did blood flow to the myometrium, endometrium, and uterine caruncles (p less than 0.05). Distribution of uterine blood flow was unaltered. Cervical and vaginal blood flows increased ten-to twenty-fold with E1 and E2alpha, while mammary gland blood flow rose 54 and 77 per cent. Significant increases in adrenal and thyroid blood flows were documented. The only significant decrease in blood flow occurred in the pancreas (- 13 percent; p less than 0.05) following E2alpha infusion. At the dose studied, E1 and E2alpha are potent vasodilators of reproductive tissues, especially the cervix and vagina. As they appear to be the major estrogens during ovine pregnancy, it is likely they participate in the preparation of these tissues for parturition.