1. The induction of phase I and II enzymes in the liver of the male F344 rat drinking 2% (w/v) solutions of green or black tea for 6 weeks was investigated. Also studied were glutathione (GSH) and cyst(e)ine in blood, liver and kidney, as well as serum cholesterol, HDL cholesterol, triglycerides, and total and free testosterone. 2. The total carbon monoxide-discernible liver P450, b5 and NADPH-cytochrome c(P450) reductase activities were similar in all groups. 3. There were significant increases in liver of rat drinking green or black tea of P4501A1, 1A2 and 2B1 activities, but no change in P4502E1 and 3A4 activities. Of the phase II enzymes, UDP-glucuronyltransferase was increased, but glutathione S-transferase was not. 4. Serum GSH was higher in the group administered black tea, but GSH and cyst(e)ine in other groups was at control levels. Serum cholesterol was lower in rat given black compared with green tea. Triglycerides had a declining trend after green and black tea exposure compared with water controls. Free and total testosterone were not affected. 5. Thus, beverages widely used by man altered host biochemistry as regards specific phase I and II enzymes in liver of rat and specific serum parameters.