In cardiac-specific Na⁺-Ca²⁺ exchanger (NCX) knockout (KO) mice, the ventricular action potential (AP) is shortened. This, along with as a decrease of the L-type Ca²⁺ current (I_Ca), provides a critical mechanism for the maintenance of Ca²⁺-homeostasis and contractility in the absence of NCX (Pott et al., 2005, Circ Res,97,1288). To investigate the mechanism that underlies the accelerated AP repolarization, we recorded the transient outward current (I_to) in patch-clamped myocytes isolated from wildtype (WT) and NCX-KO mice. Peak I_to was increased by 78% and decay kinetics were slowed in KO versus WT. Consistent with increased I_to, electrocardiograms (ECG) from KO mice exhibited shortened QT-intervals. Expression of the I_to-generating K⁺ channel subunit Kv4.2 and the K⁺-channel-interacting-protein (KChip) were increased in KO. We used a computer model of the murine AP (Bondarenko et al.,2004, Am J Physiol,287,1378) to determine the relative contributions of increased I_to, reduced I_Ca, and reduced Na⁺-Ca²⁺ exchange current (INCX) on the shape and kinetics of the AP. Reduction of I_Ca and elimination of I_NCX had relatively small effects on the duration of the AP in the computer model. In contrast, AP repolarization was substantially accelerated when I_to was increased in the computer model. Thus, the increase in I_to, and not the reduction of I_Ca or I_NCX, is likely to be the major mechanism of AP shortening in KO myocytes. The upregulation of I_to may comprise an important regulatory mechanism to limit Ca²⁺ influx via a reduction of AP-duration, thus preventing Ca²⁺-overload in situations of reduced myocyte Ca²⁺ extrusion capacity.