In a therapeutic trial, the effect of short-term low-dosageN-acetylcysteine supplementation on glutathione metabolism was investigated in two patients with hereditary glutathione deficiency (5-oxoprolinuria). Clinical and neurophysiological examinations of the patients indicated progressive neurological damage. The pretreatment concentrations of total and free glutathione in leukocytes were 15–20% of normal, whereas the corresponding γ-glutamylcysteine levels were increased. In plasma, the glutathione concentrations were similarly decreased, but no γ-glutamylcysteine was detected. Total glutathione in erythrocytes was markedly decreased. Low urinary excretion of cysteinylglycine, cyst(e)ine, taurine,N-acetylcysteine, mercaptolactate and mercaptoacetate and reduced leukocyte taurine levels constituted additional evidence of decreased intracellular availability of cysteine, i.e. glutathione. Oral supplementation withN-acetylcysteine (5 mg/kg × 3/day) had no effect on acid-base balance, erythrocyte glutathione levels or 5-oxoproline concentrations in plasma and urine. In leukocytes, the glutathione concentrations were increased by 20–30%, whereas the γ-glutamylcysteine levels were essentially unaltered. In parallel, the urinary excretion of cysteinylglycine was increased and the leukocyte levels and urinary outputs of sulphur amino acids were restored. No side-effects of the treatment were noted. The results indicate thatN-acetylcysteine may be of value in increasing the low intracellular glutathione concentrations and cysteine availability in patients with hereditary glutathione synthetase deficiency.