Prazosin hydrochloride, a systemic antihypertensive agent, lowers intraocular pressure when applied topically to normal rabbit eyes. A 0.01% solution of topically administered prazosin did not alter out-flow facility, episcleral venous pressure, systemic blood pressure, or ocular blood flow. Tonography suggested a 27% decrease in the rate of aqueous humor formation two hours following topical administration. Posterior chamber aqueous ascorbate levels were increased following topical administration of prazosin, which also suggested a decreased entry rate of water into the eye. The effect of prazosin on IOP was prevented by systemic pretreatment with phentolamine mesylate, an alpha-adrenergic blocking agent, but not by propranolol hydrochloride or atropine sulfate. This was consistent with the known action of prazosin as a postsynaptic alpha-adrenergic blocking agent.