The effect of topical 2.5% phenylephrine hydrochloride, an alpha adrenergie agonist, on the rate of aqueous humor flow was studied in the eyes of 22 normal human subjects. The drug and a placebo were administered by random assignment from coded containers with the fellow eye serving as the control. Intraocular pressure, anterior chamber volume, corneal thickness and the rate of clearance of topically applied fluorescein from the anterior chamber were measured. Four of the 22 subjects underwent a study of the effect of phenylephrine on the permeability to fluorescein of the blood-aqueous barrier. No statistically significant effect of the drug was found on the rate of aqueous humor flow, intraocular pressure, anterior chamber volume, corneal thickness or permeability of the blood-aqueous barrier.