Abstract It has been reported that nitric oxide (NO) is involved in the relaxation mechanism of ginsenoside saponin in various smooth muscle in experimental animals. Although ginsenoside Rg 3 showed both endothelium-dependent and -independent component relaxation in vascular smooth muscle, the action mechanism of the relaxation of corporal muscle is not clear. We, thus, investigated the relaxation mechanism of ginsenoside Rg 3 using isolated canine corpus cavernosum. Ginsenoside Rg 3 concentration-dependently relaxed the canine corpus cavernosum that had been contracted by phenylephrine (PE), in which IC 50 was 1.68 × 10 −5 g/ml. Ginsenoside Rg 3 significantly (P < 0.05) potentiated acetylcholine (ACh)-induced relaxation in endothelium intact corpus cavernosum. Methylene blue (MB) but not N ω-nitro-L-arginine methylester (L-NAME) or ODQ (1H-[1,2,4]oxadiazol-[4,3-]quinoxsalin-1-one) modified the dose-response curve of ginsenoside Rg 3. Ginsenoside Rg 3 also significantly potentiated relaxation response to UV light in the presence of streptozotocin (STZ), which was almost completely (P < 0.01) blocked by ODQ. Ginsenoside Rg 3 concentration-dependently inhibited corporal phosphodiesterases (PDE), which resulted in increase of cyclic adenosine monophosphate (cAMP) as well as cyclic guanosine monophosphate (cGMP) contents in corporal smooth muscles. MB inhibited the accumulation of cGMP but not cAMP by ginsenoside Rg 3. These results indicate that mechanism responsible for the relaxation by ginsenoside Rg 3 is not by stimulating endothelial nitric oxide synthase (eNOS) of the canine corporal smooth muscle but by increasing cyclic nucleotide levels through PDE inhibition.