We have produced mice in which expression of the rat cardiac Na⁺/Ca²⁺ exchanger (NCX1) transgene was switched on when doxycycline was removed from the feed at 5 weeks. At 8 to 10 weeks, NCX1 expression in induced (Ind) mouse hearts was 2.5-fold higher but protein levels of sarco(endo)plasmic reticulum Ca²⁺-ATPase, 1- and 2-subunits of Na⁺-K⁺-ATPase, phospholamban, ryanodine receptor, calsequestrin, unphosphorylated and phosphorylated phospholemman were unchanged when compared to wild-type (WT) or non-induced (Non-Ind) hearts. There was no cellular hypertrophy since WT, Non-Ind and Ind myocytes had similar whole cell membrane capacitance. In Ind myocytes, NCX1 current amplitude was ~42% higher, L-type Ca²⁺ current amplitude was unchanged, and action potential duration was prolonged when compared to WT or Non-Ind myocytes. Contraction and [Ca²⁺]_i transient amplitudes in Ind myocytes were lower at 0.6, not different at 1.8, and higher at 5.0 mM [Ca²⁺]_o when compared to WT or Non-Ind myocytes. Despite similar Ca²⁺ current amplitude and SR Ca²⁺ uptake, SR Ca²⁺ content at 5.0 mM [Ca²⁺]_o was significantly higher in Ind compared to Non-Ind myocytes, indicating that NCX1 directly contributed to SR Ca²⁺ loading . Echocardiography demonstrated that heart rate, left ventricular mass, ejection fraction, stroke volume and cardiac output were similar among the 3 groups of animals. In vivo close-chest catheterization demonstrated similar contractility (+dP/dt) and relaxation (-dP/dt) among the 3 groups of mice, both at baseline and after stimulation with isoproterenol. We conclude that induced expression of NCX1 transgene resulted in altered [Ca²⁺]_i homeostasis, myocyte contractility and action potential morphology. In addition, heart failure did not occur 3 to 5 weeks after NCX1 transgene was induced to be expressed at levels found in diseased hearts.