In the present study, ciprofloxacin, a fluoroquinolone antibiotic, was evaluated in vivo. The toxic effects of ciprofloxacin (20 and 200 mg/kg/day) on kidney and liver functions, and on chromosomes were investigated in rats. 20 mg/kg/day ciprofloxacin did not significantly affect urine composition, serum enzyme levels and the haematocrit, affecting only creatinine clearance. The 200 mg/kg/day dose increased urine volume, serum creatinine and and creatinine clearance. Ciprofloxacin at both doses had no significant numerical effect on chromosomes, but there was a statistically significant dose-dependent increase in chromatid-type breakage. Significant histological changes in the liver and kidney were not observed.