Objective:To investigate the change of CSX/Nkx2.5 gene mRNA expression and its protein location in the developing rat hearts. Methods:Embryonic hearts of normal rats were obtained on the 12th,15th,17th and 19th day during embryonic period. Reverse transcription polymerase chain reaction(RT-PCR) was used to reveal CSX/Nkx2.5 gene mRNA expression,and Immunohistochemistry for its protein location. The results were analyzed by the statistical method of One-way ANOVA. Results:With the development of rat hearts,the expression levels of CSX/Nkx2.5 gene mRNA were up-regulated. CSX/Nkx2.5 protein was mainly expressed in the atria ,ventricles and trabecular muscles,while it was not detected in the endocardium, epicardium, valvular tissues, outflow tracks and large arteries. Conclusion:CSX/Nkx2.5 gene is up-regulated in the rat heart and specifically expressed in the nucleus of cardiac muscle cell. It suggests that CSX/Nkx2.5 should play an important role in the fetal heart development.