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.