Concentrations of nucleic acids and protein were measured in the oviducts of ovariectomized mice following injection regimes chosen to induce both synergism and antagonism between estradiol and progesterone in the uterus. In the first experiment, a 4-day series of injections produced marked antagonism by progesterone of the estradiol-induced synthesis of DNA, RNA, and protein in the uterus. Whole-organ concentrations of RNA and protein were increased in the oviduct by estradiol, but these increases were not antagonized by progesterone. The 7-day series of injections used in the second experiment produced significant but not marked synergism with regard to DNA, RNA, and protein concentrations in the uterus. No significant effects of these injections were observed in the oviduct, with the exception of an apparent suppression of RNA and protein synthesis induced by prolonged treatment with progesterone. The mouse oviduct thus emerges as an organ whose genomic regulation by hormones is considerably different than that occurring in the uterus. DNA content of the oviduct could not be altered by any experimental treatment. Furthermore, with regard to the normal mouse reproductive cycle, it seems probable that most, if not all, of the effects of progesterone on oviductal function, either directly or in concert with estradiol, are mediated by mechanisms independent of those involved in the genomic regulation of RNA and protein synthesis, at least as assessed by whole-organ concentrations of nucleic acid and protein.