Apraclonidine hydrochloride (ALO 2145), a clonidine derivative that does not cross the blood-brain barrier, was applied topically to one eye of each of 20 normal human subjects. The rate of aqueous humor flow and the permeability of the blood-aqueous barrier were measured by fluorophotometry. Four hours after administration, the flow rate in the apraclonidine-treated eyes was 35% lower than that measured in the control eyes. Three hours after instillation, the intraocular pressure (IOP) was 34% lower in the apraclonidine-treated eyes when compared with control eyes. Both these differences were statistically significant. The drug had little, if any, effect on blood-aqueous permeability.