Abstract: Objective To explore the optimal condition for the differentiation of rat bone marrow mesenchymal stem cells (BMSCs) into cardiomyocyte-like cells EM>in vitro /EM>by using oxytocin（OT）. Methods SD rat BMSCs were isolated and cultured from rat bone marrow, then induced by OT for 96 hours. The cultured cells were observed by phase-contrast microscope. The immunohistochemical technique and laser scanning confocal microscope (LSCM) were used for detecting of expression of desmin, α-sarcomeric actin and cardiac troponin T(C-TnT). The induced cells were evaluated by a transmission electron microscope. Results The　morphology of BMSCs induced by OT changed obviously. After induced by OT for one week, the cells became larger and extended, and most of them became long shuttle-shape and paralleled. After four weeks, most cells became rod-shape and touched with adjacent cells to form myotube-shape. BMSCs induced by OT could be identified by the positive staining for desmin,α-sarcomeric actin and C-TnT. Desmin, α-sarcomeric actinpositive cells made up higher of all BMSCs than cTnT-positive cells. Desmin and α-sarcomeric actin-positive cells were about 33.24% and 58.06%, cTnT-positive cells were about 28.32%. laser scanning confocal microscopy indicated that α-sarcomeric actin was signed red, cTnT was green.When they were both been showed,it changed to yellow.Transmission electron microscope showed that the induced cells had a cardiomyocyte-like ultrastructure: a lot of mitochondria，rough endoplasmic reticulum and ribosome were founded in plasm, and paralleled m

Key words: Marrow mesenchymal stem cell, Oxytocin, Cardiomyocyte, Differentiation, Immunohistochemistry, Rat