Ca²⁺-Calmodulin (Ca²⁺-CaM)-dependent protein kinase II (Ca²⁺/CaMKII) is an important regulator of cardiac ion channels and its inhibition may be an approach for treatment of ventricular arrhythmias. We investigated the role of W-7, an inhibitor of Ca²⁺-occupied CaM, and KN-93, an inhibitor of Ca²⁺/CaMKII, on Kv4.3, in Xenopus laevis oocytes using the two-electrode voltage-clamp technique. W-7 caused a voltage- and concentration-dependent decrease in peak current with an IC₅₀=92.4µM. The block was voltage-dependent with an effective electrical distance of 0.18±0.05 and use-dependence was observed, suggesting that a component of W-7 inhibition of I_Kv4.3 was due to open-channel block. W-7 made recovery from open-state inactivation a bi-exponential process, also suggesting open-channel block. We compared the effects of W-7, with those of KN-93 following washout of 500µM BAPTA-AM. KN-93 reduced peak current without evidence of voltage or use dependence. Both W-7 and KN-93 accelerated all components of inactivation. We used wild type and mutated Kv4.3 channels with mutant CaMKII consensus phosphorylation sites to examine the effects of W-7 and KN-93. In contrast to W-7, KN-93 at 35 µM selectively accelerated open-state inactivation in the WT vs. the mutant channel. W-7 had a significantly greater effect on recovery from inactivation in WT vs. mutant channels. We conclude that at certain concentrations KN-93 selectively inhibit Ca²⁺/CaMKII activity in Xenopus oocytes and the effects of W-7 are mediated by direct interaction with the channel pore and inhibition of Ca²⁺-CaM as well as a change in activity of Ca²⁺-CaM-dependent enzymes including Ca²⁺/CaMKII.