Objective Current treatment for cardiac bradyarrhythmias is implantable mechanical pacemakers. We investigate the feasibility of stem cell therapy for serious bradyarrhythmias by transplantation of ES cell-derived cardiac myocytes into the AV nodal region of mice with complete AV block. Methods Mice complete AV block were created in 8-10 week-old ICR mice by injecting PBS containing 20-30% glycerol into the AV nodal region after the mid-sternotomy. ES cell-derived cardiac myocytes were isolated from NKx2.5–GFP knock-in ES cells-derived embryoid bodies (FASEB J. 2003:740-2). Five days after the initial operation, embryonal stem cell derived cardiac myocytes ( 2*105 cell ) were injected into the AV nodal region. ECGs were monitored after the injection of embryonal stem cell derived cardiac myocytes. AV block mice injected with PBS alone were used as control. Results The mice injected with PBS alone showed complete AV block throughout the observation period ( N=5 ). In contrast 3/5 mice injected with PBS with ES cells-derived cardiac myocytes restored AV conduction after the injection. Conclusion Transplantation of ES cell-derived cardiac myocytes restored the AV conduction in the heart with surgically created complete AV block in mice. It could be a new therapeutic approach to treat life-threatening bradyarrhythmia.