To investigate the pharmacodynamics of phentolamine in human corpus cavernosum (HCC) with special attention to the role of the K þ channels. Strips of HCC precontracted with nonadrenergic stimuli and kept in isometric organ bath immersed in a modified Krebs-Henseleit solution enriched with guanethidine and indomethacine were used in order to study the mechanism of the phentolamine-induced relaxation. Phentolamine caused relaxation (E50%) in HCC strips precontracted with K þ 40 mM. This effect was not blocked by tetrodotoxin (1 lM) (54.674.6 vs 48.976.4%) or (atropine (10 lM) (52.776.5 vs 58.675.6%). However, this relaxation was significantly attenuated by L-NAME (100 lM) (59.775.8 vs 27.877.1%; Po0.05; n ¼ 8) and ODQ (100 lM) (62.775.1 vs 26.873.9%; Po0.05; n ¼ 8). Charybdotoxin and apamin (K Ca -channel blockers) did not affect the phentolamine relaxations (54.674.6 vs 59.375.2%). Glibenclamide (100 lM), an inhibitor of K ATP -channel, caused a significant inhibition (56.776.3 vs 11.372.3%; Po0.05; n ¼ 8) of the phentolamine-induced relaxation. In addition, the association of glibenclamide and L-NAME almost abolished the phentolamine-mediated relaxation (54.675.6 vs 5.771.4%; Po0.05; n ¼ 8). The results suggest that phentolamine relaxes HCC by a nonadrenergicnoncholinergic mechanism dependent on nitric oxide synthase activity and activation of K ATPchannel.