As a precedent study for elucidating the mechanism of possible urinary bladder carcinogenesis due to xanthine crystals induced by FYX-051, a xanthine oxidoreductase inhibitor, we have determined the experimental conditions suitable for the 52-week simultaneous treatment with citrate in F344 rats. Simultaneous treatment with citrate and FYX-051 produced both increased urinary citrate excretion and suppression of urinary xanthine deposition at around 4 hours after a single dosing, but these effects disappeared 2 hours later, indicating a lack of the durability of citrate effects. Next, we carried out a 7-day simultaneous treatment study by two daily treatments, that is, FYX-051 (6 mg/kg) and citrate (2,000 mg/kg), followed by citrate-alone treatment, under the conditions of selected dosing intervals, the second dose of citrate, and dosing volume. As a result, the dosing interval of citrate was found to be optimal at 4 hours, but not at 3 or 5 hours, because this treatment completely inhibited intrarenal xanthine deposition. The dose of citrate for the second treatment and the dosing volume were found to be sufficient at 1,500 mg/kg and 10 mL/kg, respectively. Subsequently, a 4-week study by simultaneous treatment at 3 mg/kg of FYX-051 and citrate (2,000 mg/kg) + citrate (1,500 mg/kg), under the improved conditions, revealed that renal lesions could be drastically inhibited. Thus, the present study demonstrated that the interval of two citrate treatments is pivotal and indicated that the improved model would be useful for the mechanistic study of FYX-051-induced urinary bladder carcinogenesis because of an easier treatment method than our previous model.