To investigate the usefulness of allopurinol for the treatment of hyperuricemia in birds, experimental studies were performed using the physiologically occurring post-prandial hyperuricaemia in birds of prey as a model. Pre-and post-prandial plasma concentrations of allopurinol, oxypurinol, xanthine, hypoxanthine and uric acid were established by high performance liquid chromatography in red-tailed hawks (RTH, Buteo jamaicensis) at various time intervals after receiving allopurinol (50 mg/kg SID) or placebo. The dosage used caused slight, but significantly elevated plasma uric acid concentrations compared to controls, as well as vomiting in the majority of treated birds. Markedly elevated plasma concentrations of oxypurinol, xanthine and hypoxanthine were seen in experimental birds. Toxic signs were attributed to oxypurinol, the active (and toxic) metabolite of allopurinol. Xanthinuria was considered to be the cause of the observed renal function disorder. Extrapolation of data from studies in humans and combining these with those of the present study suggest that the maximum dose of allopurinol that can be safely administered to RTH is about half the dose given in the present study, but this needs verification.