Adult male Sprague-Dawley rats were fed a purified diet containing 5% Maillard browned egg albumin (EA-B) or browned hydrolysed egg albumin (HEA-B) for 10 wk. Control animals were pair-fed a corresponding isocaloric, isonitrogenous non-browned egg albumin (EA-C) or hydrolysed egg albumin (HEA-C) diet. At the end of 10 wk, the rats were killed and hepatic, small intestinal and colonic microsomes and cytosol fractions were prepared by ultracentrifugation. Animals fed EA-B exhibited significantly (P less than 0.05) increased hepatic benzo[alpha]pyrene hydroxylase activity and significantly (P less than 0.05) decreased colonic aminopyrine N-demethylase activity compared to control (EA-C) animals. HEA-B-fed animals also exhibited a significant (P less than 0.05) decrease in colonic aminopyrine N-demethylase activity compared with HEA-C controls, but no significant differences were detected in hepatic or small intestinal enzyme activities in this group. These data suggest that Maillard browned protein products may modify hepatic and/or colonic drug-metabolizing enzyme system activities, and may thus contribute to alterations in the metabolism of endogenous substrates and of exogenous drugs, precarcinogens and other xenobiotics.