In anesthetized dogs, pyrazinoate reduced the fractional excretion of urate when the concentration of the drug in plasma ranged from 3 to 300 micrograms/ml. At higher concentrations, fractional excretion rose progressively. p-Aminohippurate at concentrations in plasma ranging from 1 to 1000 micrograms/ml had no effect on the fractional excretion of urate. In guinea pigs, pyrazinoate exerted only a faint uricosuric action and p-aminohippurate was without effect. Both substances were tested over the range of 1 to 1000 micrograms/ml of plasma. The results in dogs help explain some previous reports of negative experiments with pyrazinoate; the doses used were not optimal for detecting responses. The results in guinea pigs suggested that urate secretion in this animal does not fit either of two previously recognized mechanisms.