The voltage-dependent K⁺ channel responsible for the slowly activating delayed K⁺ current I_Ks is composed of pore-forming KCNQ1 and regulatory KCNE1 subunits, which are mutated in familial forms of cardiac long QT syndrome. Because KCNQ1 and KCNE1 genes also are expressed in epithelial tissues, such as the kidneys and the intestine, we have investigated the adaptation of KCNE1-deficient mice to different K⁺ and Na⁺ intakes. On a normal K⁺ diet, homozygous kcne1^−/− mice exhibit signs of chronic volume depletion associated with fecal Na⁺ and K⁺ wasting and have lower plasma K⁺ concentration and higher levels of aldosterone than wild-type mice. Although plasma aldosterone can be suppressed by low K⁺ diets or stimulated by low Na⁺ diets, a high K⁺ diet provokes a tremendous increase of plasma aldosterone levels in kcne1^−/− mice as compared with wild-type mice (7.1-fold vs. 1.8-fold) despite lower plasma K⁺ in kcne1^−/− mice. This exacerbated aldosterone production in kcne1^−/− mice is accompanied by an abnormally high plasma renin concentration, which could partly explain the hyperaldosteronism. In addition, we found that KCNE1 and KCNQ1 mRNAs are expressed in the zona glomerulosa of adrenal glands where I_Ks may directly participate in the control of aldosterone production by plasma K⁺. These results, which show that KCNE1 and I_Ks are involved in K⁺ homeostasis, might have important implications for patients with I_Ks-related long QT syndrome, because hypokalemia is a well known risk factor for the occurrence of torsades de pointes ventricular arrhythmia.