A repeated dose toxicity study of prulifloxacin, a new antibacterial agent, was conducted in Sprague-Dawley rats. Male and female rats were given the test material orally for 4 weeks at doses of 0 (control), 30, 300 and 3000 mg/kg. After discontinuation of the treatment, a 4-week recovery test was also conducted. There was one case of death in the 3000 mg/kg group. Grayish green and soft feces, unkempt fur, transient deep respiration and decreased body weight gain were observed in the 3000 mg/kg group. Decreased food consumption and increased water intake were seen in the 300 and 3000 mg/kg groups. Ophthalmoscopic examination failed to show any abnormalities related to the treatment. In urinalysis, crystalline substance in the urinary sediments, cloudy urine and decreased Na+ excretion were observed in the 300 and 3000 mg/kg groups. Increased urine volume, lowered urine specific gravity and decreased K+ and Cl- excretions were seen in the 3000 mg/kg group. Hematologic examination showed decreased Hb, Ht, MCV and MCH and increased WBC in the 3000 mg/kg group. Blood chemical examination revealed increased BUN and decreased K+ and Cl- in the 3000 mg/kg group, and decreased K+ and gamma-globulin in the 300 mg/kg group. Pathological changes caused by the treatment were as follows. Cecal weight was increased in all dose groups. Cecal distention and swelling of its absorptive cells were seen in the 300 and 3000 mg/kg groups. In kidney, tubular nephrosis with crystalline substance was observed in the 300 and 3000 mg/kg groups, and its organ weight was increased in the 3000 mg/kg group. The above-mentioned changes were reversible except for decreased gamma-globulin, increased BUN and urine volume, and lowered urine specific gravity. Ulcer and small cavities associated with proliferation of fibrous tissue in the femoral articular cartilage were observed in the 3000 mg/kg group at the end of recovery period of 4 weeks. Plasma levels and urinary concentrations of active metabolite of the test material were increased in all dose groups with dose-related manner, whereby no sex difference was observed. No effects caused by the repeated dosing were seen in the plasma concentrations. Increased cecal weight in the 30 mg/kg group was considered to be attributable to the pharmacological effect of the test material. The results show that the NOAEL of prulifloxacin is 30 mg/kg for 4-week repeated dose toxicity in rats.