An open placebo-controlled study on eleven healthy male volunteers was carried out to investigate renal tolerance and the possibility of crystalluria after oral and intravenous administration of ofloxacin. Subjects received single doses of 200 and 400 mg ofloxacin orally, 200 mg ofloxacin intravenously, or placebo. Urine was collected in several fractions on each study day and the urine volume, excretion of ofloxacin, and excretion of creatinine, alanine aminopeptidase, N-acetyl-beta-D-glucosaminidase, and gamma-glutamyltransferase were calculated in each fraction collected. Furthermore, the urine was investigated microscopically to determine whether ofloxacin crystals were present. Serum creatinine and creatinine clearance were also calculated. No relevant changes in creatinine clearance were observed and no drug crystals were found in the urine which indicates that renal tolerance was good. There were no differences in ofloxacin recovery after oral or intravenous administration, confirming that absolute bioavailability of the oral form is excellent.