On the metallothionein, glutathione and cysteine relationship in rat liver. Stress and starvation increased liver metallothionein (MT) and decreased liver glutathione (GSH) levels. Serum cysteine plus cystine levels were increased by stress. The exogenous administration of GSH, while not modifying hepatic GSH content, increased liver MT levels in basal and starved rats but not in stressed rats. Liver and serum cysteine levels were increased by GSH administration, a process partially reverted by the irreversible inhibitor of gamma-glutamyl transpeptidase, alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid. Mouse and rat liver MT levels were also increased by buthionine sulfoximine, an inhibitor of GSH synthesis, indicating that GSH is not a necessary precursor of MT. In addition, the hepatic MT content was increased by the administration of cysteine in a dose-response manner. These results suggest that hepatic MT synthesis is elevated by increased cysteine pools, and that MT, GSH and cysteine levels are somehow inter-related. MT, besides GSH, may be contemplated as a putative intracellular reservoir of cysteine in the liver of adult rats. 