To elucidate the efficacy of Korean red ginseng (KRG) on penile erectile tissue and erectile response, the effect of ginseng was evaluated in both in vivo and in vitro experiments in laboratory animals. Fifty milligrams of KRG per kilogram in weight was mixed in physiologic saline and given by mouth for 3 months to both rabbits and rats. In vitro experiments were performed by observing the responses to acetylcholine (ACh) and KRG of strips of rabbit corpus cavernosum. In vivo experiments were performed by measuring the cavernosal pressures after the stimulation of pelvic nerves innervating rat corpus cavernosum. On rabbit cavernosal muscle strips precontracted with phenylephrine (5x10(-6) M), increasing concentrations of ACh (10(-7), 10(-6), 10(-5), 10(-4) M) showed dose dependent relaxation in the control group (10(-7) M: 15.32%, 10(-6) M: 35.44%, 10(-5) M: 59.45%, 10(-4) M: 76.54%)(p<0.01). After 3 months of KRG administration, the relaxation responses to ACh were increased significantly (10(-7) M: 34.18%, 10(-6) M: 56.35%, 10(-5) M: 75.33%, 10(-4) M: 89.86%)(p<0.01). Relaxation effects of KRG were significantly increased after administering KRG for 3 months, as evident by the intracavernousal pressure to electrostimulation being 107.52 cm. water in the control group and significantly increased to 138.34 cm. water after 3 months administration of KRG (p<0.01). We confirmed that the long-term administration of KRG enhances erectile capacity and that its action is mediated by endothelium-derived relaxing factor and peripheral neurophysiologic enhancement.