The effect of moxisylyte hydrochloride on isolated human penile corpus cavernosum tissue was investigated and compared with other a-adrenergic antagonists. Moxisylyte produced a concentration-dependent relaxation of a norepinephrine-induced (1 x 10(-5) M) contraction of the corpus cavernosum tissue. Pretreatment with 1 x 10(-6) M doses of moxisylyte reduced competitively the norepinephrine-induced contraction. The competitive effect of prazosin was strongest among four tested agents, followed by phentolamine, moxisylyte, and then yohimbine. The activity ratio for each antagonist is 2.4 for moxisylyte, as compared with 28.2 for prazosin, 6.7 for phentolamine, and 1.6 for yohimbine respectively. Moxisylyte hydrochloride is an agent with potential clinical and research uses capable of producing erection when injected intra-cavernously.