Certain evidence is presented, based on the effect of physostigmine, atropine and nicotine, that the vaso-dilation of the vessels of the penis occurring on stimulation of the dilator nerves is due to a local hormonal mechanism of the acetylcholine type, though direct proof that acetylcholine was liberated peripherally could not be obtained. They also present evidence that the action of skeletal muscles cannot obstruct the venous out-flow sufficiently to play any major part in the mechanism of erection. Contractions of the ischio-cavernosus and accelerator-urinae muscles may aid in the initiation of the process but not in its maintenance. The rise of blood pressure in the corpus cavernosum, due to the relaxation of the arterioles, is so rapid and probably so great, as to make the venous out-flow incompetent.