PURPOSE: Carbon monoxide (CO), as well as nitric oxide (NO), have been proposed as potential effectors in the non-adrenergic, non-cholinergic (NANC), nerve-mediated relaxation of the prostate. Attempts were made to determine the localization and expression of heme oxygenase-2 (HO-2), and to observe the change in the relaxation of the smooth muscle induced by CO, depending on age and castration, in rat prostate glands. MATERIALS AND METHODS: The prostate smooth muscles, isolated from young (125+/-4.5g, n=18), adult (321+/-17.8g, n=18), old (413+/-6.4g, n=18) and castrated adult (318+/-15.2g, n=18) rats, were used. The HO-2 immunohistochemistry was observed using the rabbit anti-HO-2 polyclonal antibody. The expressions of HO-2 were measured by a reverse transcriptase-polymerase chain reaction (RT-PCR) and western blotting. Polygraphy, connecting the force displacement transducer, was used to observe the relaxation effect of CO. To investigate the relaxation action of the CO mediated, to the soluble guanylate cyclase (sGC) enzyme inhibitors, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and methylene blue were added to the reaction solutions. RESULTS: The HO-2 was located in the nerve fibers of the rat prostates. A quantitative analysis of PCR products revealed greater decreased levels of HO-2 mRNA and protein expression in the young (p<0.05) and castrated adults (p<0.05) than in the normal adult rats. The relaxation effect of CO was greater in the adult than in the young (p<0.05), old and castrated adult rats (p<0.05), but the effect was inhibited by the addition of ODQ and methylene blue in all groups (p<0.05). CONCLUSIONS: CO has an effect on the relaxation of rat prostate smooth muscle. The expression of HO-2 in the prostate became higher with increasing age, so it is estimated the relaxation effect of CO in the older adults will be higher than with the other ages. Androgen deprivation decreases relaxation effect of CO in prostate smooth muscle.