In two different ovarian preparations in vivo, androstenedione-C14 was infused via the left ovarian artery of the dog. In such animals, estradiol-C14 and estrone-C14 could be isolated from the venous blood of the infused organ. If clomiphene was infused via the left ovarian artery concomitant with androstenedione-C14, the rate of conversion of androstenedione-C14 to estradiol-C14 and to estrone-C14 increased. Clomiphene, moreover, increased the ovarian production of estradiol from non-radioactive precursors. The data are discussed with regard to possible mechanism of action of clomiphene on endocrine organs.