Pretreatment of rats with methylmercury hydroxide (MMH) (15 mg/kg s.c. for 2 days) protected against hepatotoxicity due to the inhalation of CCl4 vapor (4800-6100 ppm for 2 h). This was evidenced by lessening of the changes due to CCl4 in liver glucose-6-phosphatase, serum glutamic oxalacetic transaminase (SGOT), serum glutamic pyruvic transaminase (SGPT), serum isocitrate dehydrogenase and serum sorbitol dehydrogenase. Decreases in p-nitroanisole demethylation and cytochrome P-450 were also altered. Lipid peroxidation due to CCl4 was decreased by MMH. These biochemical indices of protection were supported by histopathological observations. These results lend further support to the concept that metabolism of CCl4 is necessary for its hepatoxicity.