Introduction Insulin-like growth factor-1 (IGF-1) is one of the growth factor families.We have confirmed gene transfer of IGF-1 and intracavernosal injection (ICI) of IGF-1 protein to the penis could improve erectile capacity.In this study, we further determined the effects of ICI of IGF-1 on corpus cavernosal smooth muscle vasoreactivity in aging rats.Aim.To determine whether ICI of IGF-1 protein can improve corpus cavernosal smooth muscle relaxation in aging rats.Main Outcome Measures.Vasoreactivity,morphological changes, Nitric oxide (NO) and guanosine 3,5-cyclic-monophosphate (cGMP) formation were determined.Methods.Five young (4 months) and 15 old (24 months) Sprague-Dawley male rats were enrolled in this study.The old rats were divided into 3 groups: vehicle-only (n=5), IGF-1 1 μg/kg (n=5) and IGF-1 10μg/kg treatment group (n=5).After 4 weeks of single IGF-1 injection treatment, strips of corporal tissue were precontracted with phenylephrine, and dose-response curves were generated to evaluate endothelial-dependent (acetylcholine, ACH), endothelial-independent (sodium nitroprusside, SNP) and electrical field stimulation (EFS)vasoreactivity.The changes of percentage of cavernosal smooth muscle and the concentration of NO and cGMP in penile tissue were also evaluated.Results.After IGF-1 treatment, the vasoreactivity was significantly improved in both the 1μg/kg and the 10 μg/kg treatment group compared to the vehicle-only group at 4 weeks in response to ACH, SNP and EFS (P < 0.05, respectively).The percentage of cavernosal smooth muscle was increased in IGF-1 treatment group.The NO and cGMP concentrations were all increased after IGF-1 treatment.Conclusions:These data demonstrate that ICI of IGF-1 improved vasoreactivity in aging rat corpus cavernosum, which may restoration the tone of smooth muscle of corpus cavernosum and formation of NO and cGMP.