Gap junctional intercellular communication and expression of gap junction proteins (connexins) are decreased frequently in neoplastic cells including human ovarian carcinoma cells. In order to test the hypothesis that these changes contribute to the neoplastic phenotype of ovarian carcinoma cells, we transfected human ovarian carcinoma SKOV-3 cells with connexin43. Stable, connexin43-expressing transfectants were characterized for cell proliferation in vitro in normal, low-serum, and serum-free culture medium, for tumorigenicity in nude mice, and for sensitivity to adriamycin in vitro. Transfected clones expressed higher levels of connexin43 and gap junctional intercellular communication, reduced proliferation and greater dependence upon serum for growth in vitro, decreased tumor formation, increased sensitivity to adriamycin, and reduced expression of p-glycoprotein. These data suggest that gap junctional intercellular communication and/or connexin43 expression suppresses the neoplastic phenotype of ovarian carcinoma cells and their downregulation is involved in neoplastic transformation of ovarian epithelial cells. The increased sensitivity to adriamycin and elevated expression of p-glycoprotein by the transfected cells also suggest that gap junctional intercellular communication and connexin43 expression are involved in drug sensitivity and might be manipulated to enhance the clinical response.