Objective To investigate the expression profiles of connexin40 (Cx40) in cultured atrial muscles of newborn rats.Methods These cells were divided randomly into 36 culture dishes and 6 experiment groups.The expression of Cx40 was detected by using immunocytochemistry on the 2nd,7th,14th,21st,28th,and 35th day respectively.Results On the 2nd day after culture,Cx40 was expressed in the cardiac myocytes,mainly localized on the cell membrane and in the cytoplasma,and the positive expression rate in cell-cell was (6.2 ± 0.4 ) %.On the 7th day,the positive expression rate was ( 82.6 ±7.2) %,the expression of Cx40 on the cell membrane and in the cytoplasma was decreased and mainly localized between the junction of cell membrane.On the 14th day,at the junction of both side-side and endend,Cx40 were expressed,but no expression on the cell membrane was detected.On the 21st day,the positive expression rate was (90.1 ± 8.9 ) %,and Cx40 was expressed at intercalary disc and distributed like strip.On the 28th day,the positive expression rate was (79.8 ± 3.5 ) %,Cx40 was expressed disorderly.On the 35th day,the positive expression rate was (49.4 ±5.7)%,and Cx40 was expressed in side-intercalary disc (ID),end-ID and side-end ID.Conclusion The expression of Cx40 in the cultured atrial muscle cells of neonatal rats was concerned with the atrial muscle cellular function and changed with the cultural time. Key words: Connexin 40; Atrial muscle; Cell culture