Capsaicin-sensitive primary afferents widely innervate the genitourinary tract and play an important role in the integration of various mechanisms which activate reflexes leading to penile erection. We investigated the effect of intraurethral capsaicin infusion in psychogenically impotent men. The 20 patients were prospectively randomized to four groups, each of five men. Group A received intraurethral infusion of saline solution, group B intraurethral capsaicin (10(-5) M), group C intracavernosal papaverine hydrochloride (8 mg) plus intraurethral saline infusion, and group D intracavernosal papaverine hydrochloride (8 mg) plus intraurethral capsaicin (10(-5) M). The penile response was recorded real-time. Intraurethral capsaicin induced penile erection, as did the papaverine injection, while saline infusion was without effect. The erectile response to intraurethral infusion of capsaicin is suggested to involve activation of a urethra-corpora cavernosa reflex arc. Further studies are necessary to clarify if this arc is integrated at central nervous system level or is locally triggered, and if it may have pathophysiologic implications.