An investigation was undertaken of the effect on the purine metabolism of the rat of 2-amino-4-hydroxy-6-formyl pteridine, known to inhibit xanthine oxidase, and sodium bromide, reported to retard uricopoiesis. Neither the intraperitoneal administration of the pteridine nor the oral administration of sodium bromide significantly lowered the tissue concentration of “soluble urate” or prevented the further appearance of such urate upon incubation of the tissues. No comparable inhibition of the in vitro activity of xanthine oxidase was shown by more than 300 other compounds, many of similar structure, when tested by the Thunberg technique.