Rationale: The sodium–calcium exchanger 1 (NCX1) is predominantly expressed in the heart and is implicated in controlling automaticity in isolated sinoatrial node (SAN) pacemaker cells, but the potential role of NCX1 in determining heart rate in vivo is unknown. Objective: To determine the role of Ncx1 in heart rate. Methods and Results: We used global myocardial and SAN-targeted conditional Ncx1 knockout ( Ncx1 −/− ) mice to measure the effect of the NCX current on pacemaking activity in vivo, ex vivo, and in isolated SAN cells. We induced conditional Ncx1 −/− using a Cre/loxP system. Unexpectedly, in vivo and ex vivo hearts and isolated SAN cells showed that basal rates in Ncx1 −/− (retaining ≈20% of control level NCX current) and control mice were similar, suggesting that physiological NCX1 expression is not required for determining resting heart rate. However, increases in heart rate and SAN cell automaticity in response to isoproterenol or the dihydropyridine Ca 2+ channel agonist BayK8644 were significantly blunted or eliminated in Ncx1 −/− mice, indicating that NCX1 is important for fight or flight heart rate responses. In contrast, the pacemaker current and L-type Ca 2+ currents were equivalent in control and Ncx1 −/− SAN cells under resting and isoproterenol-stimulated conditions. Ivabradine, a pacemaker current antagonist with clinical efficacy, reduced basal SAN cell automaticity similarly in control and Ncx1 −/− mice. However, ivabradine decreased automaticity in SAN cells isolated from Ncx1 −/− mice more effectively than in control SAN cells after isoproterenol, suggesting that the importance of NCX current in fight or flight rate increases is enhanced after pacemaker current inhibition. Conclusions: Physiological Ncx1 expression is required for increasing sinus rates in vivo, ex vivo, and in isolated SAN cells, but not for maintaining resting heart rate.