A single standardized drop of 1.0% prednisolone acetate labeled with tritiated thymidine was administered topically to one eye of 58 patients shortly before elective cataract extraction. An aqueous humor sample was aspirated at varying intervals and its corticosteroid content was determined. Peak drug concentration in aqueous humor was 1.13 μg/ml, which occurred 30 to 45 minutes after instillation of the medication. Substantial quantities of corticosteroid were found in the aqueous humor five minutes after drug administration. The area under the drug concentration in aqueous humor-vs-time curve (a measure of the drug's bioavailability in aqueous humor) was 88 μg min/ ml, and its half-life in human aqueous humor was 28 minutes. None of these values were significantly different from the comparable values in rabbit eyes.