A distinct spatial and temporal pattern of connexin26 and connexin43 (cx26 and cx43) expression was observed in the rat endometrium in response to embryo implantation; however, connexin expression was suppressed during the preimplantation period. Pseudopregnant rats did not show connexin mRNA, while artificial decidualization induced by a scratch led to a strong expression of cx26 and cx43 in the endometrium of these animals. In order to examine the regulatory effects of ovarian steroid hormones on connexin expression, ovariectomized rats were treated with progesterone (P) and/or estradiol-17 beta (E2). Untreated, ovariectomized animals expressed mRNA for cx43, but not for cx26. Endometrial expression of mRNA for both connexins was strongly enhanced by E2 treatment; immunolabeling revealed protein for cx26 in the uterine luminal epithelial cells and for cx43 in the uterine stromal cells. P treatment, either alone or in combination with E2, suppressed expression of connexin mRNA. P suppression in the presence of E2 was reversible when P was withdrawn. When administered on Days 0-2 of pregnancy, the antiprogestin onapristone inhibited the effect of P and gave rise to strong expression of both connexin transcripts. These results demonstrate that expression of cx26 and cx43 in the rat uterine endometrium is differentially regulated by E2 and P during early pregnancy.