In cardiomyocytes rapidly inactivating K⁺ channels mediate a transient outward current (I_to), which underlies the initial repolarization of the action potential. I_to has two components, which differ with respect to their kinetics of recovery from inactivation: I_to,s shows slow recovery, I_to,f fast recovery from inactivation. I_to,s is mediated by Kv1.4, I_to,f by Kv4 channels (Kv4.3 in humans and mainly Kv4.2 in mouse and rat). Kv4 channels may form complexes with Kv Channel Interacting Proteins (KChIPs), which further accelerate recovery kinetics. The aim of this study was to correlate mRNA expression profiles in ventricular myocytes and their remodelling with the functional expression of individual K⁺ current components and their kinetics. For this purpose we injected cRNAs for candidate channels (Kv4.2, Kv1.4 and KChIP2) in Xenopus oocytes according to the relative amounts of mRNA detected with RT-PCR in control and hypertrophied murine ventricular myocytes. Surprisingly, we only found fast recovery from inactivation with a single-exponential time course, obviously dictated by the Kv4.2/KChIP2 channels (t= 83 ms). The recovery time constant obtained with Kv4.2 in the absence of KChIP2 was 670 ms. Notably, these two time constants, obtained in separate oocyte experiments, reflect the double-exponential recovery kinetics found in individual cardiac myocytes. We consider the possibility of distinct Kv4 channel populations in cardiac myocytes, including heteromultimeric Kv4 channels with KChIP bound and fast recovery kinetics, and Kv4 homotetramers with no KChIP bound and slower recovery kinetics.