This study was designed to determine the drug concentration and clearance after intravitreal injection of ciprofloxacin. Ciprofloxacin (100 μg/0.1cc) was injected into the vitreous of normal eyes and into eyes that had undergone lensectomy and vitrectomy. Drug concentration was determined using high-pressure liquid chromatography analysis 0.5, 2, 4, and 12 hours after injection. In normal eyes, the elimination half life was 2.2 hours with a distribution volume of 1.2 ml. In aphakic vitrectomized eyes, the half life was 1 hour and the distribution volume was 1.4 ml. The majority of the drug appears to leave via a transretinal route, but clearance was not inhibited by probenecid. Because of the relatively short half life, intravitreal administration of ciprofloxacin for the treatment of endophthalmitis would provide only short-term therapy and would need to be supplemented by other forms of treatment.