—Cardiac hypertrophy and heart failure are known to be associated with a reduction in Ca²⁺-ATPase pump levels of the sarcoplasmic reticulum (SR). To determine whether, and to what extent, alterations in Ca²⁺ pump numbers can affect contraction and relaxation parameters of the heart, we have overexpressed the cardiac SR Ca²⁺-ATPase specifically in the mouse heart using the α-myosin heavy chain promoter. Analysis of 2 independent transgenic lines demonstrated that sarco(endo)plasmic reticulum Ca²⁺-ATPase isoform (SERCA2a) mRNA levels were increased 3.88±0.4-fold and 7.90±0.2-fold over those of the control mice. SERCA2a protein levels were increased by 1.31±0.05-fold and 1.54±0.05-fold in these lines despite high levels of mRNA, suggesting that complex regulatory mechanisms may determine the SERCA2a pump levels. The maximum velocity of Ca²⁺ uptake (V_max) was increased by 37%, demonstrating that increased pump levels result in increased SR Ca²⁺ uptake function. However, the apparent affinity of the SR Ca²⁺-ATPase for Ca²⁺ remains unchanged in transgenic hearts. To evaluate the effects of overexpression of the SR Ca²⁺ pump on cardiac contractility, we used the isolated perfused work-performing heart model. The transgenic hearts showed significantly higher myocardial contractile function, as indicated by increased maximal rates of pressure development for contraction (+dP/dt) and relaxation (–dP/dt), together with shortening of the normalized time to peak pressure and time to half relaxation. Measurements of intracellular free calcium concentration and contractile force in trabeculae revealed a doubling of Ca²⁺ transient amplitude, with a concomitant boost in contractility. The present study demonstrates that increases in SERCA2a pump levels can directly enhance contractile function of the heart by increasing SR Ca²⁺ transport.