Objective To understand the mechanism of how cardiomyocytes exit from the cell cycle, we examined the expression of pololike kinase 1(plk1) in the postnatal developmental process of cardiac myocytes. Methods Mitotic Index(MI) of cardiomyocytes was examined in the neonatal, 2weekold, 4weekold, and adult rat hearts (five cases per groups) by double immunofluorescence stained with H3P and α-sarcomeric actin antibodies. plk1 mRNA and protein expression during the postnatal developmental process of cardiac myocytes were detected by RT-PCR and Western blot analysis in rat hearts. Results The MI of cardiomyocytes in 0dayold hearts (0.905±0.087%) was approximately 24 times over that in 2weekold hearts(0.372±0.094%)（P<0.01）. The mRNA expression of plk1 is obviously declined during postnatal 7 days to 10 days in rat hearts（P<0.01）, and was undetectable in the adult group.