Background: Our objectives were to assess the pharmacokinetic interaction and body fluid penetration of ciprofloxacin and levofloxacin. Methods: This study was a single-dose open randomized three-way crossover in 15 healthy volunteers receiving 500 mg oral levofloxacin, 500 mg oral ciprofloxacin, or 250 mg levofloxacin and 250 mg ciprofloxacin co-administered. Serum, urine, and body fluid concentrations were determined by high-performance liquid chromatography and analyzed via population pharmacokinetic modeling. Results: Modeling indicated that ciprofloxacin inhibited the renal reabsorption of levofloxacin. Ciprofloxacin increased the net renal clearance of levofloxacin by 13 , as its estimated affinity for a putative tubular reabsorption transporter was 12-fold higher (Km: 568 A?M) compared to levofloxacin (Km: 6,830 A?M). Levofloxacin increased the bioavailability of ciprofloxacin by 12 and achieved significantly (p <0.05) higher concentrations at 3 h in ejaculate, prostatic, seminal, and vaginal fluid compared to ciprofloxacin. Conclusion: Modeling suggested that ciprofloxacin inhibited the tubular reabsorption of levofloxacin due to a 12-fold higher affinity for a putative tubular reabsorption transporter compared to levofloxacin. This pharmacokinetic interaction was not clinically relevant.