The repolarization phase of the cardiac action potential is dependent on transmembrane K⁺ currents. The slow (I_Ks) and fast (I_Kr) components of the delayed-rectifier cardiac K⁺ current are generated by pore-forming α subunits KCNQ1 and KCNH2, respectively, in association with regulatory β-subunit KCNE1, KCNE2 and perphaps KCNE3. In the present study we have investigated the distribution of transcripts encoding these five potassium channel-forming subunits during mouse heart development as well as the protein distribution of KCNQ1 and KCNH2. KCNQ1 and KCNH2 mRNAs (and protein) are first expressed at embryonic day (E) 9.5, showing comparable levels of expression within the atrial and ventricular myocardium during the embryonic and fetal stages. In contrast, the β-subunits display a more dynamic pattern of expression during development. KCNE1 expression is first observed at E9.5 throughout the entire myocardium and progressively is confined to the ventricular myocardium. With further development (E16.5), KCNE1 expression is mainly confined to the compact ventricular myocardium. KCNE2 is first expressed at E9.5 and it is restricted already to the atrial myocardium. KCNE3 is first expressed at E8.5 throughout the myocardium and with further development, it becomes restricted to the atrial myocardium. The fact that α subunits are homogeneously distributed within the myocardium, whereas the β subunits display a regionalized expression profile during cardiac development, suggest that differences in the slow and fast component of the delayed-rectifier cardiac K⁺ currents between the atrial and the ventricular cardiomyocytes are mainly determined by differential β-subunit distribution.