The expression of two different gap junction transcripts, connexin26 and connexin43, has been investigated in rat endometrium during implantation and early chorionic-allantoic placentation. An induction of connexin26 protein is detected locally in the epithelium of the implantation chamber prior to trophoblast invasion. At the same stage connexin43 is found exclusively in the primary decidual zone. By the sixth day of pregnancy the uterine epithelium of the implantation chamber has degenerated and the invading trophoblast is directly confronted with the developing decidua. Connexin26 is then detected in decidual cells accompanying the invading trophoblast, whereas connexin43 has spread through the decidual tissue up into the myometrial layers. This distribution pattern of connexin43 and connexin26 is maintained until at least Day 9 of pregnancy. In decidual cells adjacent to the trophoblast both connexins seem to be coexpressed in the same gap junctional plaque. In these early stages of pregnancy a clear correlation exists between levels of connexin43 mRNA and protein expression. In regard to connexin26 mRNA a similar correlation is found from Day 5 of pregnancy onward. The amount of mRNA on Day 4, however, does not correspond to the amount of connexin26 antigen in the endometrium at that stage. The spatial and temporal expression pattern of connexin26 accompanies the process of embryo implantation in the surrounding endometrium, pointing to a specialized function of these cells.