Chronic oral treatment of young female Fischer rats with 25 mg/kg Δ9-tetrahydrocannabinol (THC) per day inhibited aminopyrine demethylation and significantly increased benzo[a]pyrene oxidation by the liver. THC treatment also elevated serum corticosterone levels and produced a significant loss of body weight. The weight loss was not due to vehicle or food intake (pair-feeding). Pair-feeding did, however, produce a stimulation of both mixed function oxidase pathways as well as a marked elevation in serum corticosterone levels. The results indicate that THC has a differential effect on mixed function oxidase pathways in the liver that is not directly related to food intake or corticosterone levels