The pharmacokinetics of ciprofloxacin (Bay o 9867) was examined after a single oral dose of 250 mg and a single intravenous dose of 100 mg respectively in six healthy male volunteers in an open, randomized crossover study. Although ciprofloxacin concentrations were measured in serum, plasma and urine by HPLC with fluorimetric detection and by microbiological assay, all pharmacokinetic calculations are based on the highly sensitive HPLC method only. The mean serum concentration of ciprofloxacin peaked approximately 1 h after the oral dose (0.94 mg/l). The elimination half-life was about 4 h and the renal clearance was 4.75 ml/min . kg; both were independent of the route of administration. The total clearance (9.62 ml/min . kg) was about twofold higher than the renal clearance. The volume of distribution of the central compartment was calculated to be 0.161 l/kg and the total volume at steady state was 2.0 l/kg. About 27% of the oral dose was excreted in urine, whereas the urinary recovery of the i.v. dose was 46%. The absolute bioavailability of ciprofloxacin was found to be approximately 60%. Ciprofloxacin appears to follow first-order, three compartment model kinetics.