The purpose of this study was to determine the aqueous humor concentrations of topically applied ciprofloxacin (CAS 86393-32-0), ofloxacin (CAS 82419-36-1) and tobramycin (CAS 79645-27-5). Thirty patients undergoing cataract extraction or trabeculectomy were randomly divided into three groups and each of the group received either 0.3% ciprofloxacin, ofloxacin or tobramycin topical drops preoperatively. Eyedrops were instilled for six times at a frequency of one drop every 15 minutes, beginning 90 minutes before initiation of the surgery. At the time of surgery, 0.1 ml aqueous fluid was aspirated from the anterior chamber. Concentrations of the antimicrobial agents were determined using the microbroth dilution procedure outlined by the National Committee for Clinical Laboratory Standards. Escherichia coli (ATCC 25922) was used as a standard strain for determination of minimal inhibitory concentrations (MICs). The mean aqueous levels of ciprofloxacin and ofloxacin were found to be 0.092 +/- 0.077 microgram/ml, 0.964 +/- 0.693 microgram/ml, respectively. Tobramycin did not reach the concentration that could be detected by applied method. CONCLUSIONS The mean aqueous humor levels of ofloxacin and ciprofloxacin were more than the MICs levels for most of the ocular pathogens which may cause postoperative endophthalmitis.