Cardiac myocyte contraction depends on transmembrane L-type Ca²⁺ currents and the ensuing release of Ca²⁺ from the sarcoplasmic reticulum. Here we show that these L-type Ca²⁺ currents are essential for cardiac pump function in the mouse at developmental stages where the functional significance of the heart becomes imperative to blood flow and to the continuing growth and survival of the embryo. Disruption of the Ca_vβ₂ gene, which encodes for the predominant ancillary β subunit of cardiac Ca²⁺ channels, resulted in diminished L-type Ca²⁺ currents in cardiomyocytes of embryonic day 9.5 (E9.5). This led to a functionally compromised heart, causing defective remodeling of intra- and extraembryonic blood vessels and embryonic death following E10.5. The defects in vascular remodeling were also observed when the Ca_vβ₂ gene was selectively targeted in cardiomyocytes, demonstrating that they are secondary to cardiac failure rather than a result of the lack of Ca_vβ₂ proteins in the vasculature. Partial rescue of the Ca²⁺ channel currents by a Ca²⁺ channel agonist significantly postponed embryonic death in Ca_vβ₂^−/− mice. Taken together, these data strongly support the essential role of L-type Ca²⁺ channel activity in cardiomyocytes for normal heart development and function and that this is a prerequisite for proper maturation of the vasculature.