Understanding molecular mechanisms involved in atrial tissue remodeling and arrhythmogenesis in atrial fibrillation (AF) is essential for developing specific therapeutic approaches. Two-pore-domain potassium (K 2P ) channels modulate cellular excitability, and TASK-1 (K 2P 3.1) currents were recently shown to alter atrial action potential duration in AF and heart failure (HF). Finding animal models of AF that closely resemble pathophysiological alterations in human is a challenging task. This study aimed to analyze murine cardiac expression patterns of K 2P channels and to assess modulation of K 2P channel expression in murine models of AF and HF. Expression of cardiac K 2P channels was quantified by real-time qPCR and immunoblot in mouse models of AF cAMP-response element modulator (CREM)-IbΔC-X transgenic animals or HF (cardiac dysfunction induced by transverse aortic constriction, TAC). Cloned murine, human, and porcine TASK-1 channels were heterologously expressed in Xenopus laevis oocytes. Two-electrode voltage clamp experiments were used for functional characterization. In murine models, among members of the K 2P channel family, TASK-1 expression displayed highest levels in both atrial and ventricular tissue samples. Furthermore, K 2P 2.1, K 2P 5.1, and K 2P 6.1 showed significant expression levels. In CREM-transgenic mice, atrial expression of TASK-1 was significantly reduced in comparison with wild-type animals. In a murine model of TAC-induced pressure overload, ventricular TASK-1 expression remained unchanged, while atrial TASK-1 levels were significantly downregulated. When heterologously expressed in Xenopus oocytes , currents of murine, porcine, and human TASK-1 displayed similar characteristics. TASK-1 channels display robust cardiac expression in mice. Murine, porcine, and human TASK-1 channels share functional similarities. Dysregulation of atrial TASK-1 expression in murine AF and HF models suggests a mechanistic contribution to arrhythmogenesis.