The catalytic activity of purified glutathione-insulin transhydrogenase (thiol:protein-disulfide oxidoreductase/isomerase, EC 1.8.4.2) from bovine pancreas is markedly stimulated by histidine and other chelating agents. The activation produced was highest with EDTA, followed by EGTA, 8-hydroxyquinoline and 1,10-phenanthroline. Of the many amino acids tested, histidine was the only one that activated the enzyme; the structurally related compounds, 3-methylhistidine and imidazole also stimulated the enzyme, but 1-methylhistidine and histamine were without effect. The activation of EDTA was negated by metal ions, most effectively by Se2+, Hg2+, Cu2+ and Zn2+, and less effectively by Ca2+ and Ni2+. Likewise, activation by histidine was negated by Zn2+ but not by Ca2+ or Mg2+. Thus, activation of glutathione-insulin transhydrogenase is apparently achieved in part by the chelation of inhibitory metal ion(s). These findings are consistent with a regulatory scheme for glutathione-insulin transhydrogenase in which (a) the enzyme is inhibited by selenium and heavy metal ions normally present in tissues and (b) this inhibition can be relieved by the addition of histidine or chelating agents.