1. Trophoblast invasion during embryo implantation in some aspects resembles tumour cell invasion but, unlike tumour cells, trophoblast cells are able to differentiate and establish a placenta. Because direct cell-cell communication is believed to be involved in growth control and differentiation, we have investigated connexin (cx) gene expression during trophoblast development. 2. Pre-implantation embryos expressed cx43 as well as cx31 proteins from the 8-cell stage onwards. Following implantation, compartmentalization of both connexins occurred: cx31 expression was restricted to the invasive trophoblast cell population, whereas the embryo proper was characterized by cx43. Trophoblast differentiation was indicated by induction of cx26 in the labyrinth and cx43 in the spongiotrophoblast accompanied by a disappearance of cx31. Comparison with trophoblast cell lines revealed that rat trophoblast HRP-1 cells express connexin43, while malignant choriocarcinoma cells express cx31. Treatment with retinoic acid led to a disappearance of cx31 in the choriocarcinoma. Both cell lines reduced their invasion properties after retinoic acid treatment, but growth retardation was only observed in the malignant trophoblast. 3. It seems that the cx31 channel is needed for trophoblast cell populations to maintain the highly proliferative properties but does not alter their invasion properties.