Channels responsible for slowly activating delayed-rectifier potassium current (I_Ks) are composed of KCNQ1 and KCNE1 subunits, and these channels play a role in the repolarization of cardiac action potentials. Recently, we showed that the antihyperlipidemic drug probucol, which induces QT prolongation, decreases the I_Ks after 24-h treatment. In the present study, we investigated the effects of three cholesterol-lowering agents (probucol, an enhancer of cholesterol efflux; simvastatin, a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor; and triparanol, a 3β-hydroxysterol-24-reductase inhibitor) on cholesterol synthesis, the KCNQ1 current (I_KCNQ1), and the I_Ks to clarify the differences in the modes of action of these agents on the I_Ks. Probucol did not inhibit cholesterol synthesis and had no effect on I_KCNQ1, while I_Ks decreased after 24-h treatment. Simvastatin inhibited cholesterol synthesis and decreased I_KCNQ1 and I_Ks. Additionally, the activation kinetics of I_Ks became faster, compared with that of control I_Ks. Triparanol inhibited cholesterol synthesis but did not reduce I_KCNQ1 and I_Ks. However, the activation kinetics of I_Ks became faster. Our data indicated that the mechanism by which probucol inhibits I_Ks was not mediated by the inhibition of cholesterol synthesis but depended on an interaction with the KCNQ1/KCNE1 complex. Meanwhile, the reduction in cholesterol induced by simvastatin and triparanol is one of the mechanisms that affects the kinetics of I_ks.