The effects of known uricosuric agents have been examined by standard clearance techniques in male Cebus monkeys. Probenecid was effective in promoting uricosuria at p.o. doses greater than 10 mg/kg. Sulfinpyrasone was only mildly active in this species. Zoxazolamine, at p.o. doses sufficient to produce marked uricosuria in man, was inactive. Phenylbutasone was weakly active; carinamide did not elicit a uricosuric response under our experimental conditions. Iodopyracet at high loads was markedly uricosuric. Chlorothiaside at i.v. doses of 25 mg/kg elicited a biphasic response of uricosuria followed by urate retention. The relative inactivity of certain uricosuric agents in the Cebus monkey is thought to be due to either a basic difference from man in the urate transport system or the result of rapid biotransformation to an inactive metabolite(s), poor oral adsorption or a combination of these.