Rats given a dose of carbon tetrachloride (CCl4) immediately received injections of glucagon and insulin every 4 h. They frequently died after 4 h and showed a significantly higher mortality between 8 h and 28 h as compared to the control rats where such deaths occurred 16 h later. At 8 h, the derangements of SGPT values and prothrombin time were significantly greater in the hormone-treated rats than in the control rats. In these CCl4-intoxicated rats, hepatic reduced glutathione content at 4 h was significantly reduced after hormone treatment. The treatment significantly enhanced CCl4 metabolism, conversion of 14CCl4 into 14CO2 in vitro, by microsomes isolated from the liver, whereas it did not affect the microsomal cytochrome P450 content. These results suggest that glucagon and insulin treatment increased CCl4 hepatotoxicity in rats through activating the cytochrome P450-dependent mono-oxygenase system. This would merit consideration for the clinical application of this treatment.