When rats were fed a diet containing 0.4% (w/w) butylated hydroxytoluene (BHT), a three-fold increase in total glutathione (GSH) S-transferase activity towards 1-chloro-2,4-dinitrobenzene (CDNB) was observed in liver but not in lung or kidney. Hepatic GSH S-transferase activities towards styrene oxide (SO) and 1,2-epoxy-3-(p-nitrophenoxy)propane (EPNP) were also increased, but to a lesser extent. Isoelectric focusing studies indicated that the activities of most of the rat liver GSH S-transferase isoenzymes were induced. Immunoprecipitation studies of the native and induced enzymes suggested that de novo synthesis of these proteins caused the increase in GSH S-transferase activity in liver. A two-fold increase in glutathione reductase activity in liver upon dietary administration of BHT was observed. Kinetic and physical properties of the native and induced enzymes were similar which may indicate that the induction is due to the synthesis of this enzyme. A significant increase in reduced glutathione (GSH) content in liver and lung was also seen in rats treated with BHT.