TheKCNE1gene encodes a channel regulator IsK which in association with the KvLQT1 K⁺channel protein determines the slow component of the cardiac delayed rectifier current. We have investigated the cellular electrophysiological characteristics of adultKCNE1-knockout mouse hearts by means of the standard microelectrode technique. Action potential parameters from the ventricular endocardium ofKCNE1−/− mice were indistinguishable from those ofKCNE1+/+ animals. In particular,KCNE1−/− hearts did not exhibit prolonged repolarization. E-4031, a specific blocker of erg K+ channels consistently prolonged repolarization inKCNE1+/+ but not inKCNE1−/− hearts. By contrast, the chromanol compound 293B, a specific blocker of KvLQT1 K+ channel produced comparable effects on repolarization inKCNE1−/− andKCNE1+/+ mice. We conclude that invalidation of the mouseKCNE1gene by homologous recombination leads to a mild cardiac phenotype at the cellular level.