The relationship between hepatic glutathione content and hepatic glutathione conjugation rate in the rat in vivo was investigated. As substrate for glutathione conjugation, racemic (R,S)-2-bromoisovalerylurea (BIU) was used which gives rise to the biliary excretion of two diastereoisomeric glutathione conjugates and the urinary excretion of two diastereoisomeric mercapturates. The excretion rate of the glutathione conjugate in bile reflects hepatic conjugation exclusively. An intravenous infusion of BIU was given and the excretion rates of the metabolites in bile and urine were determined. The glutathione concentration in the liver was followed by taking biopsies every hour. Glutathione was depleted by the infused substrate; in rats that were pretreated with the inhibitor of glutathione biosynthesis, buthionine sulphoximine (BSO), the depletion of the glutathione content was more rapid. The rate of excretion of the glutathione conjugate in bile was plotted against hepatic glutathione content. These results indicate that the 'organ Km' for glutathione in the liver is approximately 0.5 mumol/g of liver, so that the hepatic glutathione conjugation rate is decreased only at severe glutathione depletion.