T-type Ca2+channels may play a role in cardiac development. We studied the developmental regulation of the T-type currents ( ICa,T) in cardiomyocytes (CMs) derived from mouse embryonic stem cells (ESCs). ICa,Twas studied in isolated CMs by whole cell patch clamp. Subsequently, CMs were identified by the myosin light chain 2v-driven green fluorescent protein expression, and laser capture microdissection was used to isolate total RNA from groups of cells at various developmental time points. ICa,Tshowed characteristics of Cav3.1, such as resistance to Ni2+block, and a transient increase during development, correlating with measures of spontaneous electrical activity. Real-time RT-PCR showed that Cav3.1 mRNA abundance correlated ( r2= 0.81) with ICa,T. The mRNA copy number was low at 7+4 days (2 copies/cell), increased significantly by 7+10 days (27/cell; P < 0.01), peaked at 7+16 days (174/cell), and declined significantly at 7+27 days (25/cell). These data suggest that ICa,Tis developmentally regulated at the level of mRNA abundance and that this regulation parallels measures of pacemaker activity, suggesting that ICa,Tmight play a role in the spontaneous contractions during CM development.