• Two doses of ciprofloxacin were administered intravenously, 200 mg every 12 hours, to 25 patients undergoing cataract surgery. Plasma and aqueous humor were obtained at 1, 3, 5, 7, and 9 hours after the administration of the second dose of the drug. Peak intraocular concentrations (mean ± SD), 0.21 ±0.1 mg/L, were detected at one hour following ciprofloxacin administration. A time-dependent decrease of the penetration was observed, and by nine hours after the administration, ciprofloxacin levels were 0.05 mg/L. These results illustrate that ciprofloxacin may be an effective antimicrobial agent for prophylactic use in ophthalmologic surgery and also for the treatment of intraocular infections due to susceptible organisms.