We investigated the hemodynamic changes induced in the canine corpora cavernosa by intracorporeal injection of papaverine or phentolamine. In six dogs studied, papaverine profoundly increased the resistance to venous outflow, whereas phentolamine (an alpha-adrenergic blocker) had little or no effect. In four other dogs, flow within the internal pudendal artery was measured with an ultrasonic flow probe placed directly on the artery. Papaverine increased arterial flow by 300 to 700 per cent over baseline levels, whereas the increase with phentolamine was only 50 to 100 per cent. We conclude that papaverine, by virtue of its direct smooth-muscle-relaxing properties, has a dual hemodynamic effect: it decreases the resistance to arterial inflow, thereby allowing large increases in the flow of blood into the penis during tumescence; and increases the resistance to venous outflow. Phentolamine only decreases the resistance to arterial inflow, more modestly than papaverine, and does not increase the resistance to venous outflow.