Objective:To test the action of uricase to lower plasma urate in hyperuricemic rat model developed with oxonate. Methods: Potassium oxonate was given via intraperitoneal injection. Uricase after the filtration of bacteria was given via caudal vein injection. Blood samples were taken from orbital vein to determine equivalents of competitive inhibitors on uricase by their alteration on uricase Vm/Km and plasma uric acid levels by a direct kinetic uricase method. Results:After the continuous administration of potassium oxonate to rats once a day for one week

plasma uric acid levels were kept over 0.40 mmol/L

meanwhile uricase-inhibiting substances were kept at steady high levels. To hyperuricemic rats after continued administration of potassium oxonate for 3 weeks

the injection of 0.5 U uricase modified with monoether poly (ethylene glycol)reduced plasma uric acid levels by paired t-test analysis with an average degree less than 12%. The injection of unmodified uricase or lower levels of modified uricase produced no expected action. Conclusion:Because of serious interference from the uricase inhibitor in rat in vivo

hyperuricemic rats developed with uricase inhibitors show limited reliability for evaluating the pharmacological actions of uricases.