The effects of corynanthine tartrate, a selective alpha 1-adrenergic receptor antagonist, were studied on intraocular pressure by pneumatonometry, outflow facility by tonography, and aqueous humor flow by fluorophotometry in laboratory animals. Unilateral topical administration of 5% corynanthine significantly lowered mean IOP (+/- SEM) in rabbits for at least six hours, in ten awake monkeys for six hours, and in ten monkeys anesthetized with ketamine for four hours. The maximum effect in awake monkeys occurred two hours after drug administration, from 15.8 +/- 0.5 mm Hg to 12.7 +/- 0.5 mm Hg, with no substantial change in control eyes. No change in outflow facility was demonstrated in 11 monkeys two hours after 5% corynanthine administration. Aqueous humor flow rates did not change in 12 monkeys up to three hours after drug administration. Corynanthine may act by increasing uveoscleral outflow.