The ciprofloxacin efficacy was compared to that of tobramycin in an Escherichia coli pyelonephritis model in rat. Treatments started 48 h after ligation of the left ureter and inoculation of the bladder and continued for 5 days. Ciprofloxacin (2.5 mg/kg/d and 10 mg/kg/d) was administered intravenously either in a single daily dose or in 2 divided doses at 12 h intervals. Tobramycin (2.5 mg/kg/d and 10 mg/kg/d) was administered by the intramuscular route, in a single daily dose. Ciprofloxacin 10 mg/kg/d was as efficacious as tobramycin irrespective of dosage schedule. Ciprofloxacin 2.5 mg/kg/d was more effective when given twice a day than once.