Estradiol-17beta (E2) has been considered the most potent of the natural estrogens in terms of uterine growth and vasodilatation. As a result of studies of uterine growth in rodents, estriol (E3) has been considered an "impeded" estrogen. In this study, direct comparisons of the actions of E2 and E3 on the sheep uterus were made in nonpregnant, oophorectomized ewes by using bilateral uterine artery catheters and electromagnetic flow transducers. Intra-arterial injections of E2 or E3 had equal growth-promoting and vascular effects. There were slight but statistically significant differences in the time course of vasodilatation. Estriol induced a more prolonged dilatation than did estradiol.