Erectile dysfunction (ED) is a relevant medical and psychological problem for males, and is usually associated with increased age. There is substantial evidence available suggesting that noadrenergic and opiate transmitter systems are involved in regulation of sexual function. The objective of the present study is to evaluate the effects and safety of a potential novel treatment combination for ED consisting of both, the alpha-adrenoreceptor antagonist yohimbine, and the opiate antagonist naltrexone. The study uses a three-phase, double-blind, placebo-controlled cross-over design. Healthy, male volunteers will participate. The effects of differing dosages of yohimbine (10 mg versus 20 mg) and a fixed dose of naltrexone (50 mg) on penile tumescene and rigidity, and sexual arousal are assessed in a placebo-controlled design. The anticipated risks of the study are considered minimal in relation to the importance of our increased knowledge about regulation of physiological and psychological aspects of regulation of sexual function and dysfunction. This novel treatment combination could be later also applied to patients with ED of different origin, e.g. organic, psychogenic, and mixed causes.