We studied the in vitro effect of prolactin on the synthesis of prostaglandins PGE and PGF2 alpha in the rat uterus. Uterine tissue was obtained from ovariectomized adult rats with or without estradiol-17 beta treatment. Prolactin (4 IU/ml) enhanced PGE uterine synthesis. At higher concentrations (10-15 IU/ml) prolactin enhanced PGE levels and diminished PGE2 alpha production by uterine tissue from ovariectomized rats. When ovariectomized rats were injected with estradiol-17 beta, we observed a pattern of PGE and PGF2 beta uterine similar to that of ovariectomized rats. However, the stimulatory action of PRL on PGE synthesis was augmented in the estrogenized animals. These results led us to conclude that1)-PRL exerts a luteotrophic action by regulating uterine tissue PGE synthesis, 2-estradiol-17 beta potentiates this action.