Male cats were anesthetized with pentobarbital. In one series of experiments a Foley catheter was placed suprapubically in the urinary bladder and physiologic saline, under a constant head of pressure, was allowed to flow at a constant rate through the bladder. Alpha-adrenergic stimulants such as ephedrine, norepinephrine, or methoxamine all caused a decrease or cessation in urinary flow. This effect was reversed by the alpha-adrenergic blocking agent, phentolamine. In addition, phentolamine administration caused penile erection. Erection also occurred after administration of the alpha-adrenergic blocking agent, phenoxybenzamine, or the beta2-adrenergic stimulants terbutaline or salbutamol. In each case the erection could be reversed, and usually terminated completely, by administration of the beta-adrenergic blocking agent, propranolol. Hypotension per se was not the required event to initiate erection because administration of acetylcholine, histamine, or prostaglandin E2 did not result in penile erection. In two of five experiments nitroprusside caused an erection. It was concluded that a predominance of beta2 activity in the penile vasculature is involved in erection.