The voltage-gated potassium channel K 7.1 is regulated by non-pore forming regulatory KCNE ß-subunits. Together with KCNE1, it forms the slowly activating delayed rectifier potassium current I . However, where the subunits assemble and which of the subunits determines localization of the I -complex has not been unequivocally resolved yet. We employed trafficking-deficient K 7.1 and KCNE1 mutants to investigate I trafficking using the polarized Madin-Darby Canine Kidney cell line. We find that the assembly happens early in the secretory pathway but provide three lines of evidence that it takes place in a post-endoplasmic reticulum compartment. We demonstrate that K 7.1 targets the I -complex to the basolateral membrane, but that KCNE1 can redirect the complex to the apical membrane upon mutation of critical K 7.1 basolateral targeting signals. Our data provide a possible explanation to the fact that K 7.1 can be localized apically or basolaterally in different epithelial tissues and offer a solution to divergent literature results regarding the effect of KCNE subunits on the subcellular localization of K 7.1/KCNE complexes.