To investigate the role of ras genes in trophoblastic cell lineage, we transfected the viral H- or K-ras oncogene into a human choriocarcinoma cell line, CCI, and analysed the biological properties of CCI cells expressing an activated ras oncogene. All v-H-ras-expressing clones distinctively formed the hemispherical domes, which represents an in vitro morphological expression of vectorial transport function and are characteristic of the polarized epithelial cells, but none of v-K-ras-expressing clones and control clones did. Microscopic observation demonstrated that those domes were cavities filled with fluid which accumulated between the cell layer and the surface of culture dish. Scanning electron microscopy revealed that the domes were aggregates of round cells with long numerous microvilli and were morphologically similar to a blastocyst. Furthermore, Na(+)-K(+)-ATPase activity, which is associated with the vectorial fluid transport in transporting epithelial cells, was significantly higher in the v-H-ras-expressing clones than that in the v-K-ras-expressing clones and the parental cells. Those domes flattened within 24 h after treatment with a specific inhibitor of Na(+)-K(+)-ATPase, ouabain, and the number of domes decreased in dose-dependent manner, indicating that Na(+)-K(+)-ATPase activity was required for maintainance of domes. These results suggest that up-regulated activity of H-ras but not of K-ras facilitates the vectorial fluid transport through a chorionic cell layer and leads to the dome formation. The function of II-ras in trophoblasts, may therefore, be essential for embryogenesis, especially for supplying the nutrients.