In this study, we have used Ki‐67 and MF20 mAb to determine how extensively cardiomyocytes proliferate in the postnatal mouse heart. It was established that the cardiomyocytes divided rapidly in 2‐day‐old hearts. However, at 13 days, the majority of cardiomyocytes had entered into terminal growth arrest and differentiation. We exploited this finding in order to identify proteins that were associated with cardiomyocyte growth and differentiation. The protein profiles of 2‐ and 13‐day‐old hearts were established by two‐dimensional electrophoresis and compared. Seventeen protein spots were found to be differentially expressed at day 13. Eight of them were up‐regulated while the remaining nine protein spots were down‐regulated. We focused our attention on 2 of the proteins identified by MALDI‐TOF MS, cyclin I and p53, because they are both believed to be involved in cell cycle regulation. Western blot analysis confirmed that both proteins were positively up‐regulated in the 13‐day‐old postnatal heart. To determine directly whether these proteins were associated with cell proliferation, we examined their expression patterns in H9c2 cardiomyocytes maintained in vitro. We established that cyclin I expression was low during the growing phase of H9c2 culture and high during the growth arrest/differentiation phases. In contrast, p53 expression was unchanged during both phases. The various growth phases were confirmed by the presence of cyclin A and growth arrest‐specific 1 proteins. We investigated whether silencing cyclin I expression using cyclin I‐siRNA could promote an increase in H9c2 cell proliferation. It was determined that silencing cyclin I could enhance a small, but significant, increase in H9c2 cell division. Similar results were obtained for cardiomyocytes extracted from 13‐day‐old hearts. These results imply that the reason why cardiomyocytes in 13‐day‐old hearts increased cyclin I expression was probably associated with terminal growth arrest. However, the increase in p53 expression was probably associated with cardiomyocyte differentiation, rather than growth arrest.