In order to study the effects of raising the hypoxanthine concentration in plasma on its metabolism and renal handling, the effects of intense exercise have been investigated in a patient with xanthine oxidase deficiency. Despite the 90-fold increased concentration of hypoxanthine in plasma above resting levels in normal individuals, the intracellular concentration of the initial product of hypoxanthine in cells, IMP, was unaffected. Evolution may have stabilized intracellular nucleotide concentrations against the large fluctuations in plasma hypoxanthine which occur during exercise. The renal handling of hypoxanthine is consistent with 'filtration'. In contrast, xanthine clearances may exceed those for creatinine and urinary concentrations do not correlate with those for creatinine; 'secretion' may be involved. Xanthine excretion may reflect guanine breakdown. A retrospective survey of urate concentrations in blood from 47 420 patients followed by further selected investigations detected 2 women with persistent marked hypouricaemia and high urinary urate clearances, 'Dalmatian' hypouricaemia. High pressure liquid chromatographic analysis of plasma extracts can distinguish xanthine oxidase deficiency from other causes of hypouricaemia.