Single-dose and repeated dose toxicity studies of prulifloxacin, a new antibacterial agent, were conducted in aged beagle dogs. I. A single-dose toxicity study Prulifloxacin was administered orally to aged female dogs at a single dose of 2500 and 5000 mg/kg. No death occurred in any group. Vomiting was observed in one of two animals at 2500 mg/kg and in both animals at 5000 mg/kg 3-4 hr after dosing. At 5000 mg/kg, vomiting was observed in both animals after feeding on the day after dosing. One animal also showed soft stool. Thereafter, no abnormalities were observed in any animal. No test article related changes were noted in food consumption, water consumption, body weight or pathological examination in any group. The results show that the lethal dose of prulifloxacin is judged to be greater than 5000 mg/kg in aged female dogs. II. A repeated dose toxicity study Aged male and female dogs were given the test article orally for 4 weeks at doses of 0 (control), 20, 100 and 500 mg/kg. No death occurred in any group. At 500 mg/kg, vomiting was observed every day or intermittently throughout the dosing period and salivation was observed almost every day from day 6 to the end of the dosing onward. Decreases or lack of food and water consumption, and decrease of body weight were noted at 500 mg/kg. At 100 mg/kg, slight decreases in food consumption and body weight were noted in the females. No abnormalities were noted in ophthalmoscopic or electrocardio-graphic examination. In urinalysis, decreases in Na+, K+ and Cl- concentrations and the total excretion amount were noted mostly at 500 mg/kg. A low specific gravity was noted in males at 500 mg/kg. In hematology and serum biochemistry, high GPT, BUN and creatinine, and decreases in WBC were noted in both sexes at 500 mg/kg. A high GOT was noted in males, and low Cl- in females at 500 mg/kg. At 100 mg/kg, a high GPT was noted. Rough surface in the kidney and chronic interstitial nephritis (basophilic change of tubule, atrophy of tubule, thickening of tubular basement membrane, interstitial mononuclear cell infiltration, interstitial focal fibrosis) were increased at 500 mg/kg. No toxicological findings were seen in the 20 mg/kg group. The results show that the NOAEL of prulifloxacin is 20 mg/kg for 4-week repeated dose toxicity in aged dogs.