Malignant ovarian epithelial tumors have been shown to have decreased inhibin production relative to activin production compared with normal ovarian surface epithelial (OSE) cells and nonmalignant ovarian tumors. Activin stimulates proliferation of many ovarian cancer cell lines. Inhibin antagonizes the action of activin, and inhibin-deficient mice develop gonadal tumors, suggesting that inhibin may be a tumor suppressor. However, its effects on OSE and ovarian cancer cells are unknown. We hypothesize that activin and inhibin are important regulators of biological activity in ovarian cancers. We found that inhibin A decreased murine OSE proliferation, whereas activin A had no effect. Activin A increased the proliferation of four of eight ovarian cancer cell lines (SKOV3, OCC1, OVCAR3, and A2780-s). Inhibin A decreased the proliferation of SKOV3, A2780-s, and OVCAR3 but had no effect on OCC1, ES-2, HEY, A2780-cp, and OVCA429 cells. When injected into nude mice, the inhibin-resistant cancer cell lines resulted in shorter survival time compared with the inhibin-responsive cells. Further investigations on SKOV3 and OCC1 cells showed that activin A increased invasion through Matrigel. Inhibin A decreased both basal and activin-induced proliferation and invasion of SKOV3 but had no effect on OCC1 cells. Reverse transcription-PCR analyses showed that the SKOV3 and OCC1 cells produced activin, but only SKOV3 produced inhibin. Analysis of the activin/inhibin signaling pathways indicated that Smad anchor for receptor activation was elevated in SKOV3 and OCC1 cells and that an up-regulation of the activin receptor expression may explain the inhibin resistance of OCC1 cells. Our results suggest that activin responsiveness may be gained during transformation of OSE cells and that inhibin resistance may contribute to the aggressive behavior of ovarian cancer cells.