This study elucidates the routes of elimination of ciprofloxacin and its metabolites in two groups of 5 subjects each, one of healthy volunteers, the other of patients with severe renal failure having a creatinine clearance of 12 ml/min (range 8–16 ml/min). Each subject received one dose of 200 mg ciprofloxacin infused intravenously over 30 min. In an effort to recover the total dose administered, all urine and faeces were collected for the 7 days following dosing. Blood was collected at set intervals after dosing. Serum, urine, and faeces were assayed by high-pressure liquid chromatography for ciprofloxacin and metabolites. The ciprofloxacin serum half-life in healthy volunteers was 3.9 ± 0.4 h and in patients with marked renal failure 11.2 + 2.5 h. The total amount of ciprofloxacin recovered in urine fell by a multiple of 3.4 from ± 10.7% in healthy subjects to 19.0 ± 15.9% in patients with renal failure, and the metabolites from 12.2 ± 2.3% in the former group to 5.8 ± 5.1% in the latter. In contrast, the amount of ciprofloxacin eliminated in faeces increased, by a similar factor, from ± 2.6% in healthy subjects to 37.2 ± 12.5 % in patients with renal failure. The amount of metabolites in faeces increased analogously from 7.3 ± 1.6 to 26.2 ± 6.5%. Since ciprofloxacin was administered intravenously and biliary elimination of the drug and its metabolites is negligible, we propose that elimination by faeces is due primarily to transintestinal elimination. This study demonstrates that transintestinal elimination of ciprofloxacin serves as an extrarenal safety factor compensating for reduced elimination by the renal route.