The intracavernosal pressure changes induced by 3 vasodilating drugs, diazoxide, glyceryltrinitrate and verapamil were investigated during autologous extracorporeal penile blood perfusion in an experimental rabbit model. In order to separate the arterial and venous components the experiments were performed with both intact and interrupted penile arterial inflow. The major hemodynamic effect of these drugs was an increased outflow resistance from the corpora cavernosa, probably effected through relaxation of the corporal smooth muscle. However, the intracorporeal pressure responses only reflected a part of the drug-induced smooth muscle relaxation. A theory is presented, according to which the corpora cavernosa are 'accumulating' smooth muscle relaxation, without direct reflection in an increased cavernosal pressure. At the moment when sufficient smooth muscle relaxation is achieved, a sudden increase in cavernosal pressure will appear and lead to an erection. This concept was generated through an approximation of corpora cavernosa to an elastic cylinder following the elastic law of Laplace in combination with the hydrodynamic law of Poiseuilles.