Being able to induce controlled erection in dogs and monkeys, we investigated the hemodynamics and mechanism of penile erection. 'Chronic' monkey models, having had electrodes implanted around the cavernous nerves for electroerection, were studied to evaluate the details of the hemodynamic changes. The studies included: 1) arterial blood flow, 2) corporeal pressure, 3) blood gases, 4) venous flow and 5) radiography. Tumescence of the corpora cavernosa was found to be a result of: 1) active relaxation of the sinusoidal spaces, 2) active arteriolar dilatation and 3) active venous outflow constriction. At full erection there is adequate but reduced blood flow into and out of the corpora cavernosa for metabolic exchange.