We examined the role of dietary lipids in regulating the activities and amounts of epoxide hydrolase, UDP-glucuronosyltransferase and glutathione S-transferase in rat liver. Male Wistar rats were fed a fat-free (FF) diet or isocaloric control diet containing 5% corn oil (CO) or 5% fish oil (FO) for 3 weeks. The activities of these enzymes were approx. 2-fold higher in rats fed the FO diet vs. the FF diet. Intermediate levels of enzyme activity were found in rats fed the CO diet. Diet-induced differences in enzyme levels were shown by immunoblotting. The highest levels of epoxide hydrolase, UDP-glucuronosyltransferase and glutathione S-transferase were detected in rats fed the FO diet. The lowest levels of these enzymes were found in rats fed the FF diet. Intermediate levels of enzyme were found in rats fed the CO diet. Thus, diet-induced differences in enzyme activities were paralleled by changes in enzyme levels. Fatty acid analysis of microsomal lipids showed that the FF diet was associated with decreased levels of n-6 fatty acids vs. the CO diet. The FO diet resulted in increased levels of n-3 fatty acids vs. the other diets.