Pregnant rats were injected with 3H RU38486 into the uterine cavity on day 2, 4, 5 and 8 of pregnancy. Before implantation, the saturable RU binding increased significantly (day 2 vs. 4; 30.0 +/- 2.3 vs. 640.0 +/- 17.9 dpm/g, p less than 0.001). After implantation, the binding became higher at the nidation site (implantation vs. non-implantation site; 1156.1 +/- 237.1 vs. 595.0 +/- 24.3, p less than 0.001, day 5). The dexamethasone suppressive RU binding, on the other hand, showed no significant increase during day 4, 5 and 8. Daily administration of RU38486 during day 4 and 8 caused failure to implant or to develop normal conceptus. The endometrium in these RU treated animals was stained for tissue glycogen and studied light microscopically. The tissue glycogen content appeared to be markedly decreased in RU treated animals but only at the site of implantation. The decrease was observed in both decidua and trophoblasts but not in myometrium or decidual epithelium. The present results indicate; (1) In rats, endometrial progesterone binding increases prior to implantation. (2) After implantation, the nidation site further increases its saturable progesterone uptake compared to the non-implantation site. (3) When normal process of implantation is inhibited by RU38486 administration, there occurs a decrease in the glycogen synthesis at the nidation site, including decidua and trophoblasts.