The effects of estrone (lμg/day) and prolactin (200 μg/day), alone and in combination, on deciduomal growth in response to uterine trauma and on uptake and metabolism of pi'ogesterone by nontraumatized uteri have been investigated in ovariectomized, progesteronetreated (2 mg/day) rats. Estrone significantly increased the size of traumatized uteri, whether administered during the 3-day period of uterine sensitization, immediately before traumatization, or during the 3-day period of deciduomal growth, immediately after traumatization. Prolactin significantly increased the size of traumatized uteri in the absence of estrone, but, when administered in the presence of estrone, significantly decreased the stimulatory effect normally produced by estrone. Both estrone and estradiol increased the uptake of progesterone by isolated uterine strips, but these increases could be accounted for entirely on the basis of the uterotrophic effects of the steroids and were not evident when expressed per mg uterine weight. Net accumulation (retention) of progesterone taken up by uterine tissue was markedly decreased by estradiol, due to increased rate of ring-A reduction of progesterone by uterine tissue, to 5α-pregnan-3,20-dione and 3α-hydroxy-5α-pregnan-20-one. Prolactin did not alter this uterine metabolism of progesterone in the estradiol-treated rats, but caused a moderate decrease in the stimulation of metabolism by estrone in the progesterone-treated rats. The stimulation of uterine progesterone metabolism by estrogens did not depend on increased intracellular transport of progesterone; stimulation was even more pronounced when possible cell permeability barriers were eliminated by homogenization of uteri, suggesting increased activity of the ring-A reducing enzyme system as a result of estrogenic stimulation. (Endocrinology89:191, 1971)