To elucidate the hemodynamic changes during erection, we measured corporeal blood gases in 6 monkeys before, during, and after erection induced by either papaverine or phentolamine or a combination of the two. Papaverine alone caused a strong erection (maximal tumescence and rigidity) by means of a rapid, large increase in pO2 and pCO2 with a pH drop to the acidic range. Phentolamine alone caused ‘delayed’ tumescence with less rigidity; the intracorporeal pO2 level increased, but pCO2 and pH values did not change significantly. The combination of both drugs offered no advantage over papaverine alone. We conclude that papaverine is a potent erection-inducing drug that acts in a bimodal manner, namely, it increases the arterial inflow and, at the same time, decreases the venous outflow. Phentolamine affects the arterial component of erectile function only.