Parenteral glutathione has therapeutic potential for targeted delivery of cysteine equivalents. Thus, high doses of reduced glutathione (GSH) protect from the nephrotoxic and urotoxic effects of cisplatinum and oxazaphosphorines. In order to elucidate the underlying mechanisms the kinetics and the effect of glutathione on plasma and urine sulphydryls were studied in 10 healthy volunteers. Following the intravenous infusion of 2 g m-2 of glutathione the concentration of total glutathione in plasma increased from 17.5 +/- 13.4 mumol l-1 (mean +/- SD) to 823 +/- 326 mumol l-1. The volume of distribution of exogenous glutathione was 176 +/- 107 ml kg-1 and the elimination rate constant was 0.063 +/- 0.027 min-1 corresponding to a half-life of 14.1 +/- 9.2 min. Cysteine in plasma increased from 8.9 +/- 3.5 mumol l-1 to 114 +/- 45 mumol l-1 after the infusion. In spite of the increase in cysteine, the plasma concentration of total cyst(e)ine (i.e. cysteine, cystine, and mixed disulphides) decreased, suggesting an increased uptake of cysteine from plasma into cells. Urinary excretion of glutathione and of cyst(e)ine was increased 300-fold and 10-fold, respectively, in the 90 min following the infusion. The present data suggest that the concentration of sulphydryls in the urinary tract and, more importantly, the intracellular availability of cysteine increase markedly following parenteral glutathione. The high intracellular concentration of cysteine may protect against cisplatinum and oxazaphosphorine toxicity either directly or indirectly by supporting the synthesis of glutathione.