1. Hypoxanthine, xanthine, uric acid, allopurinol and oxipurinol have been determined in skeletal-muscle biopsies by quantitative high-resolution mass spectrometry.
2. The results obtained in six untreated gout patients and in seven gout patients who had been treated with the xanthine oxidase inhibitor allopurinol for periods of about 2 years, have been compared with those obtained on muscle tissue from non-gouty subjects and with the results of previous microscopical studies of the same biopsies.
3. Measurable amounts of xanthine were detected more frequently in the allopurinol-treated gout patients than in the untreated patients. The concentration of uric acid was generally lower in the allopurinol-treated than in the untreated gout patients' muscle; and all except one of the allopurinol-treated subjects' tissue contained a measurable amount of oxipurinol. Allopurinol was detected less frequently than oxipurinol.
4. The concentrations of hypoxanthine and xanthine in the allopurinol-treated patients' muscle tissue are very much less than those which have been reported in congenital xanthine oxidase deficiency.
5. It is concluded that allopurinol can still be recommended as a useful drug in the treatment of gout but that longer studies during the clinical use of the drug would be of value.