Background: In the myocardium, myocyte cell division is irreversibly blocked shortly after birth. The signal that initiates cell cycle withdrawal is unknown. The purpose of this study was to relate changes in expression of β1 integrin and its associated α subunits to cardiomyocyte cell cycle progression during the fetal-to-neonatal developmental transition in rat. Methods and results: The developmental expression pattern and function of β1 integrin and several of its associated α subunits were examined using reverse transcription (RT) polymerase chain reaction (PCR) and β1 blocking antibodies. During the fetal to neonatal transition, a dramatic shift occurred in the levels of β1 and α isoforms. At the 17-day fetal stage only β1A was present, which remained relatively constant until immediately after birth then decreased by 30% at the adult stage. By contrast, β1D appeared at fetal day 18, increased at neonatal day 2, and afterwards remained constant. This resulted in a ratio of β1A to β1D of about 1:1 in the adult heart. The integrin β1-associated subunits, α3, α6, and α7, were expressed at extremely low levels in 17-day fetal cardiomyocytes. After birth α3 and α6 transiently increased at the 2-day neonatal stage, while α7 isoforms B, C, and X2 progressively increased to the adult stage. Unlike skeletal muscle cells, fluorescence-activated cell sorting analysis (FACS) showed no down regulation of the α5β1 fibronectin receptor during cell cycle withdrawal. Treatment of cultured cardiomyocytes with β1 blocking antibody inhibited the cell cycle in fetal but not in neonatal cells. Conclusion: These results suggest that progression through the cardiomyocyte cell cycle may be dependent upon cell attachment via integrin β1 and correlate with changes that occur in β1 spliced variants and their respective α isoforms.