PT-141, a cyclic heptapeptide melanocortin analog, was evaluated following subcutaneous administration to healthy male subjects and to patients with erectile dysfunction (ED) who report an inadequate response to Viagra. An inadequate response was defined for this study by patient report indicating that achievement of an erection suitable for vaginal penetration occurred < or =50% of the time while taking 100 mg Viagra. Erectile responses were assessed by RigiScan in healthy subjects in the absence of visual sexual stimulation (VSS) and in ED patients in the presence of VSS. Doses ranging from 0.3 to 10 mg were administered to healthy male subjects, resulting in a statistically significant erectile response at doses greater than 1.0 mg. ED patients were treated with placebo, 4 or 6 mg PT-141 in a crossover design in the presence of VSS. The erectile response induced by PT-141 was statistically significant at both doses. PT-141 was safe and well tolerated in both studies. The erectogenic potential of PT-141, its tolerability profile and its ability to cause significant erections in patients who do not have an adequate response to a PDE5 inhibitor suggest that PT-141 may provide an alternative treatment for ED with a potentially broad patient base.