Abstract: Objective: To study whether the grafted fetal cardiomyocytes could survive,proliferate and connect with the adult host myocardium. Methods: Cardiomyocytes from F344 fetal rat hearts were grafted into the hearts of adult syngenic recipient mice.The hearts were harvested and processed for histologic and electron microscopic examinations 10 days to 8 weeks after the cell grafts. Results: At 4 weeks,the grafted cardiomyocytes possessed abundunt mature mitochondria and well\|organized myofibrils compared with those at 10 days.Intercalated disks between graft and host cardiomyocytes was observed.The connection consisted of fascia adherents and gap junctions. At 8 weeks,the grafted cardiomyocytes oriented to align themselves parallel to the direction of host myocardium. Conclusion: The grafted fetal cardiomyocytes could differentiate into mature cardiomyocytes and form functional syncytia with the host myocardium in the host heart after cell implantation and the implanted cells might contract synchronously to allow effective cooperation with the host myocardium.