Previous metabolic studies of captopril suggest that the rapid dissociation of captopril-plasma protein conjugates in vivo is dependent upon endogenous thiols such as glutathione and cysteine. Consistent with this hypothesis, we have found that cysteine (0.06-3 mM) and glutathione (0.02-1 mM) cleave 14C-captopril-plasma protein conjugates in vitro. Dissociation of the drug-protein conjugate was accompanied by formation of the corresponding mixed disulphide which indicates that the reaction proceeds via a spontaneous thiol-disulphide interchange. Administration of high doses (50-300 mg/kg) of CP produced a time-dependent and dose-dependent decrease in hepatic glutathione concentrations in the mouse and the rat. The depletion of glutathione observed was similar to that produced by equimolar doses of D-penicillamine and paracetamol. Acute and chronic (7 days) administration of captopril (100 mg/kg) produces the same (11-12%) depletion of hepatic glutathione. However, changes in liver function as determined by elevation of serum glutamic-pyruvic transaminase was only observed at doses of 200 and 300 mg/kg. Thus, although thiol-disulphide interactions between captopril and plasma proteins may contribute to the perturbation of hepatic glutathione concentrations, it is unlikely that this process will be of toxicological significance during therapeutic administration of captopril.