Objective To investigate the effects of oxytocin (OT) on the differentiation of P19 cells into cardiomyocytes in vitro. Methods The P19 cells were cultivated in the medium containing different concentrations (1×10SUP>-7/SUP>,3×10SUP>-7/SUP> and 5×10SUP>-7/SUP>mol/L) of OT in suspension in dishes laid with a thin layer of agar for 4 days. Then the embryonic bodies (EBs) were transferred to coverslips in 24 well plates, allowed to adhere and be cultivated in the medium without OT for 10 to 12 days. The methods of immunocytochemical staining,transmission electron microscope(TEM) were used to identify cell differentiation. Results Some border cells in the peripheral outgrowth of the experiment groups became elongate and radial, and they were αsarcomeric actin (αSCA) and cardiac troponin T(cTnT) positively stained. The positive cells percentages the experiment group 1 (1×10SUP>-7/SUP>mol/L OT) were higher than those of other groups (P<0.01). The results of TEM indicated that the differentiated cells had the cha