Previous study in this laboratory revealed that estrogen stimulates synthesis and secretion of 97K glycoprotein [uterine secretory protein-1 (USP-1)] in rat uterine epithelial cells. In the present communication, the effects of progesterone and antiprogesterone (RU 38486) on estrogen-stimulated USP-1 biosynthesis were investigated. Ovariectomized rats were treated with estrogen for 4 days to induce USP-1 synthesis and secretion. Then, progesterone and RU 38486 were injected sc. After 6 and 12 h of treatment, uterine luminal fluid proteins were labeled for 6 h by direct administration of [35S]methionine into uterine lumen. By radioimmune precipitation assay using specific antibody against USP-1, it was shown that progesterone can reduce the USP-1 synthesis and secretion, which were prestimulated by estrogen. This antagonistic effect of progesterone on estrogen-induced USP-1 biosynthesis was also evident by the decreased staining intensity of USP-1 in uterine epithelial cells, as revealed by the immunohistochemical method. Although RU 38486 itself did not manifest any effect on estrogen-stimulated USP-1 biosynthesis, it completely inhibited the progesterone-induced decrease in synthesis and secretion. This system is believed to provide a useful model to analyze the progesterone and antiprogesterone actions on rat uterine epithelial cells.