AE1 (SLC4A1), the plasma membrane Cl^-/HCO₃^- exchanger of erythrocytes, is also expressed in heart. The aim of this study was to assess the role of AE1 in heart function, through study of AE1 null mice, which manifest severe hemolytic anemia resulting from erythrocyte fragility. Heart weight/body weight ratios were significantly higher in the AE1^-/- mice than in wild type (AE1^-/-) littermates at both 1-3 days postnatal (3.01 ± 0.38 versus 1.45 ± 0.04) and at 7 days postnatal (9.45 ± 0.53 versus 4.13 ± 0.41), indicating that loss of AE1 led to cardiac hypertrophy. Heterozygous (AE1^-/-) mice had no signs of cardiac hypertrophy. Morphology of the adult AE1^-/- mutant heart revealed an increased left ventricular mass, accompanied by increased collagen deposition and fibrosis. M-mode echocardiography revealed dysfunction of the AE1^-/- hearts, including dilated left ventricle end diastole and systole, and expanded left ventricular mass in comparison with AE1^-/- hearts. Expression of pH_i regulatory mechanisms in the hypertrophic myocardium of neonate AE1^-/- mutant mice was indistinguishable from heterozygous and control mice, as assessed by quantitative real time RT-PCR. Confocal immunofluorescence revealed that in normal mouse myocardium AE1 is sarcolemmal, while AE3 and slc26a6 are found both at the sarcolemma and in internal membranes (T-tubules and sarcoplasmic reticulum). These results indicate that AE1^-/- mice, which suffer from severe hemolytic anemia and spherocytosis, display cardiac hypertrophy and impaired cardiac function, reminiscent of findings in patients with hereditary abnormalities of red blood cells. No essential role for AE1 in heart function was found.