In order to clarify the mechanism of hyperuricemia and hyperuricosuria resulting from rapid infusion of fructose in man, the effects of an intravenous infusion of 125-200 g of fructose given over 3-4 hr on the rate of purine synthesis de novo was measured in one individual with osteoarthritis and four patients with gout. The incorporation of 1-minus 14C glycine into urinary uric acid was measured, and the pool size and turnover of urate were assessed by renal excretion of simultaneously administered 15-N urate. Fructose caused an expansion of body urate pool in all subjects, while urate turnover was increased in four. The rate of incorporation of 14-C glycine into urinary uric acid corrected for extrarenal disposal was increased in all cases (21%-430%). In two patients, rates of incorporation of 14-C glycine into urinary creatinine were increased by 10% and 11%, while rates of incorporation into uric acid were increased 84% and 159%, respectively, as a result of fructose infusion. Specific enhancement of the rate of purine synthesis de novo was suggested by these findings. The rate of infusion appeared more important than total dose in determining the magnitude of this effect. Whether the increased rate of purine synthesis was a result of direct stimulation by a fructose metabolite or was secondary to fructose-induced purine nucleotide depletion is uncertain, since the kinetics of glycine incorporation were consistent with either mechanism. Erythrocyte PP-ribose-P concentrations, however, were diminished during infusion rather than increased as might be expected if fructose infusion stimulated purine synthesis by increasing availability of this regulatory substrate.