TASK-1, a member of the recently identified K2P channel family, is mainly expressed in the heart and the nervous system. TASK-1 is regulated by several physiological and pathological conditions and functions as a background potassium channel. However, there are limited data concerning the significance of TASK-1 in cardiac physiology. We studied the functional role of TASK-1 in the heart by cardiac phenotyping the TASK-1-deficient mouse (TASK-1^−/−). TASK-1 was predominantly expressed in the ventricles of control animals. Real-time PCR and immunoblot demonstrated that the expression of seven other K2P channels was unchanged in TASK-1^−/− mice. No structural or functional abnormalities were found by histology and echocardiography. Electrophysiological studies recording monophasic action potentials (MAPs) showed a significant prolongation of action potential duration in spontaneously beating and atrially paced hearts, respectively. Surface ECGs of TASK-1^−/− mice revealed a significant prolongation of the rate corrected QT interval. Telemetric ECG recordings for 24 h, during physical and pharmacological stress testing and after ischemia/reperfusion injury did not result in a higher incidence of arrhythmias. Infarct size was comparable in both genotypes. However, TASK-1^−/− mice had a higher mean heart rate and significantly reduced heart rate variability (HRV). Time and frequency domain measurements as well as baroreceptor reflex testing revealed a sympathovagal imbalance with a shift to an increase in sympathetic influence in TASK-1^−/− mice. In conclusion, TASK-1 plays a functional role in the repolarization of the cardiac action potential in vivo and contributes to the maintenance of HRV.