AbstractUsing five‐to eight‐week‐old male F344 rats and a high‐fat (23.5% corn oil) modified AIN‐76A diet, we examined the effects of dietary restriction (a 3‐wk 30% reduction of food intake with respect to ad libitum‐fed controls) or complete fasting (2 days without food) on the activities of hepatic xenobiotic metabolizing enzymes in vitro and on azoxymethane. (AOM) induced formation of C6‐methylguanine and 7‐methylguanine in liver and colon DNA in vivo. Compared with ad libitum‐fed rats, fasting increased total liver cytochrome P450 by 32%, microsomal aniline hydroxylase by 270%, N‐nitrosodimethylamine demethylase by 270%, and azoxymethane hydroxylase by 320%. Liver benzo[a]pyrene (BP) hydroxylase and glutathione‐S‐transferase were decreased by 39% and 21%, respectively, whereas NADPH cytochrome c reductase and UDP glucuronyltransferase were unchanged. DNA methylation in the livers of fasted animals was 20–31% greater six hours after a 15 mg/kg sc injection of AOM than in ad libitum‐fed controls, whereas DNA methylation in the colon was slightly lower. In three‐week diet‐restricted animals, there were small but not statistically significant changes in the various enzyme activities and in AOM‐induced DNA methylation compared with the ad libitum‐fed controls, with the exception of BP hydroxylase, which showed a 26% decrease. However, the trends in the increase or decrease of each parameter, although small in magnitude, were similar to those observed in the case of fasting, suggesting that the effects might become significant if the duration of diet restriction were prolonged. The enhancement of A OM metabolism in rat liver by fasting, leading to increased liver DNA methylation, is different from that produced by chemical inducers, such as ethanol, where no increase in liver DNA methylation is observed.