The Aqueous humor concentration of norfloxacin was measured following topical administration in 20 eyes. The substance was applied at different time intervals. Nineteen samples obtained prior to cataract extraction had mean concentrations between 0.014 and 0.105 microgram/ml. These concentrations are under the MIC90 of most germs. One eye with minimal congestion due to a neovascular glaucoma, however, had a concentration of 0.206 microgram/ml.