Aim: The sarco(endo)plasmic reticulum calcium ATPase 2 (SERCA2) transports Ca²⁺ from cytosol into the sarcoplasmic reticulum (SR), thereby maintaining the intracellular store of Ca²⁺ necessary for contraction. Reduced SERCA function is associated with heart failure. Loss of SERCA2 would be expected to cause immediate severe myocardial contractile dysfunction and death. Results: We show that heart function can be sustained in adult mice with an inducible cardiomyocyte-specific loss of the Serca2 gene (SERCA2KO). Four weeks after induction of Serca2 gene excision, < 5 % SERCA2 protein was found in SERCA2KO myocardium compared to SERCA2FF controls. SERCA2, SERCA1 and SERCA3 protein were not detected in isolated cardiomyocytes. Strikingly, SERCA2KO mice did not present clinical signs of circulatory failure at 4 weeks. Fractional shortening was preserved, but cardiac output was reduced to 80% of control values. However, isolated cardiomyocyte shortening was reduced in SERCA2KO mice during field stimulation. Ca²⁺ transients were markedly smaller and the rate of Ca²⁺ decline was much slower in SERCA2KO than in SERCA2FF cells. The releasable SR Ca^{2 +} content in SERCA2KO cells was 27% of SERCA2FF values. The integrated L-type Ca²⁺ current and Na⁺,Ca²⁺ exchange (NCX1) current were increased in SERCA2KO cells. Likewise, expression of the L-type Ca²⁺ channel 1C and 21 subunits and NCX1 proteins were correspondingly increased in SERCA2KO myocardium. Conclusion: In SERCA2KO mice, Ca²⁺ transients large enough to sustain nearly normal heart function are generated almost completely independent of the SR by means of increased sarcolemmal Ca²⁺ fluxes.