4-Vinylcyclohexene (VCH) is present in gases discharged during synthetic rubber production. Chronic treatment of B6C3F1 mice and F-344 rats with VCH by gavage has been shown to induce ovarian tumors in mice but not in rats. Our objective was to understand the mechanism of the species difference in VCH-induced ovarian tumors. Since a critical step in the induction of ovarian tumors is destruction of the small oocyte, small oocyte counts obtained from serially sectioned ovaries were used as an index of toxicity. VCH or its epoxide metabolites [VCH-diepoxide, VCH-1,2-epoxide, and VCH-7,8-epoxide (in mice only)] were given to 28-day-old female mice and rats in corn oil, ip, at doses ranging from 0.07 to 7.4 mmol/kg body wt/day for 30 days. The dose which reduced the small oocyte count to 50% that of control was defined as the ED50. In mice, the ED50 for the reduction in small oocytes by VCH was 2.7 mmol/kg, whereas, no detectable oocyte loss occurred in rats at the highest dose of VCH (7.4 mmol/kg). The potency of the epoxides of VCH was greater than that of VCH in both species. The ED50 for oocyte loss by VCH-1,2-epoxide in mice and rats was 0.5 and 1.4 mmol/kg, respectively. In mice, VCH-7,8-epoxide had comparable potency to VCH-1,2-epoxide (ED50 = 0.7). VCH diepoxide was even more potent with ED50 values of 0.2 and 0.4 mmol/kg, in mice and rats, respectively. The dose response of the blood concentration of VCH-1,2-epoxide in mice after VCH showed that doses of VCH which caused minimal toxicity had the lowest blood level of this ovotoxic epoxide. Pretreatment of mice with the cytochrome P450 inhibitor chloramphenicol (200 mg/kg, ip) inhibited VCH epoxidation in vivo and in vitro and partially protected mice from VCH toxicity. Thus it appears that metabolism of VCH to epoxides and their subsequent destruction of oocytes are critical steps in VCH-induced ovarian tumors. Rats may be resistant to ovarian tumor induction by VCH because the amount of VCH converted to epoxides is insufficient to produce oocyte destruction.