Objective To investigate the spatio-temporal expression of CX43 i n the developing human and mouse hearts. Methods 63 normal human fetal hearts i n 6-18 gestational weeks and 64 normal mouse fetal hearts in embryonic days 9.5 -16.5 were included in this study. Immunohistochemistry was used to detect th e CX43 expression. Results In early stage of development of hu man fetal heart, no CX43 expression was detected in ventricular myocardium and w eak expression in atrial myocardium. High expression was observed in primitive t rabecular nets. With the fetal development the CX43 expression was enhanced in t he atria and ventricles. Peak expression was revealed in trabecular net at 13 - 14 weeks. Expression was weak in muscular interventricular septum. No expressio n was observed in membranous interventricular septum, atrial-ventricular valves and large arteries. Except for the large arteries, CX43 expression in the devel oping mouse heart was similar to that in human fetal hearts. Conclu- s ion CX43 may play a crucial role in the heart development.