Congenital heart defects (CHDs) occur in 0.51% of live births, yet the underlying genetic etiology remains mostly unknown. Recently, a new source of myocardial cells, namely the second heart field (SHF), was discovered in the splanchnic mesoderm. Abnormal development of the SHF leads to a spectrum of outflow tract defects, such as persistent truncus arteriosus and tetralogy of Fallot. Intracellular Ca2+ signaling is known to be essential formany aspects of heart biology including heart development, but its role in the SHF is uncertain. Here, we analyzed mice deficient for genes encoding inositol 1,4,5-trisphosphate receptors (IP3Rs), which are intracellular Ca2+ release channels on the endo/sarcoplasmic reticulum that mediate Ca2+ mobilization. Mouse embryos that are double mutant for IP3R type 1 and type 3 (IP3R1/IP3R3/) show hypoplasia of the outflow tract and the right ventricle, reduced expression of specific molecular markers and enhanced apoptosis ofmesodermal cells in the SHF. Gene expression analyses suggest that IP3R-mediated Ca2+ signalingmay involve, at least in part, theMef2CSmyd1 pathway, a transcriptional cascade essential for the SHF. These data reveal that IP3R type 1 and type 3 may play a redundant role in the development of the SHF.