The objective of this study was to evaluate the effect and mechanism of capillarisin from Artemisia capillaris (A. capillaris) on rabbit penile corpus cavernosum (PCC). The pre-contracted New Zealand White rabbit (2.5–3.0 kg) penis with phenylephrine (Phe; 10-5 m) was treated with various concentrations of ethanol extract of A. capillaris (0.1, 0.5, 1, and 2 mg/mL) and capillarisin, the active component of A. capillaris (10-7, 10-6, 10-5 and 10-4 m). Capillarisin was also applied to PCC tissues contracted with Phe, which were pre-incubated with phosphodiesterase type 5 inhibitors (PDE5 Is). Cyclic nucleotides in the perfusate were measured by radioimmunoassay. The tissues were pre-incubated with Nω nitro-l-arginine-methyl ester (L-NAME, 10-3 m) and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10-5 m) to block nitric oxide (NO) synthase and guanylate cyclase, respectively. Capillarisin induced penile relaxation and enhanced PDE5 Is-induced relaxation. Capillarisin increased cGMP and cAMP in the perfusate. The application of capillarisin on PCC pre-treated with L-NAME and ODQ significantly inhibited the relaxation. Capillarisin exerts the relaxing effect on PCC by activating the NO-cGMP and adenylyl cAMP signaling pathways and may become an alternative medicine for patients who want to use natural products to improve erectile function or do not completely respond to PDE5 Is. Copyright © 2011 John Wiley & Sons, Ltd.