The response of the uterine vascular bed to various estrogens was investigated in nonpregnant, oophorectomized sheep utilizing electromagnetic flow probes chronically implanted around the uterine arteries. One M.g estradiol-β injected directly into the uterine artery produces a maximum flow response, approximately ten times baseline levels, which is reproducible on a daily basis for periods greater than 30 days. A family of dose response curves demonstrated the following potency for increasing uterine blood flow to 50% of maximum: estradiol-β > estriol > diethylstilbestrol > estradiol-17α > estrone > ent-estradiol- 17β. (Endocrinology94: 1192, 1974)