PURPOSE: Stem cell-based cell therapy has recently been tried as a way to restore cavernosal function in an animal model. The aims of this study were to elucidate the effect of intracavernosally injected embryonic stem cells(ESCs) in aged rat. MATERIALS AND METHODS: Male Sprague Dawley rats were divided into 3 groups: young control(12 weeks old; n=10), old with vehicle injection(24 months old; n=7), and old with ESC injection(24 months old; n=8). ESCs were transfected with firefly luciferase attached to adenovirus and then injected intracavernously 2 times with a 1-week interval. Cell survival was assessed by optical molecular imaging 2 days after the last ESC injection. At 4 weeks after the last injection, intracavernosal pressure and systemic arterial pressure were recorded after pelvic nerve stimulation. Serum testosterone levels were measured by radioimmunoassay. RESULTS: We observed fluorescent signals around the external genitalia of animals injected with ESCs. The serum testosterone level of the old group(1.39+/-0.07 ng/ml) was significantly lower compared to the young control group(2.98+/-0.31 ng/ml)(p=0.03). The percentage of intracavernosal pressure/systolic blood pressure was significantly lower in the old group(58.5+/-8.6%) compared to the young control group(69.5+/-6.6%)(p=0.034). However, the old group with ESC injection(61.6+/-9.9%) did not show any significant differences from the old group(p>0.05). The old group with ESC injection showed histomorphometry similar to the old group. CONCLUSIONS: The presence of intracavernosal ESCs can be noninvasively monitored with optical molecular imaging. However, the intracavernosal injection of ESCs did not improve erectile function in the aging rat. Further studies are needed to elucidate the regulatory factors of stem cell differentiation in the corpus cavernosum.