Voltage-gated K⁺channels exist in vivo as complexes made up of pore-forming and ancillary subunits. To further our understanding of the role of a dipeptidyl peptidase-related ancillary subunit, DPP10, we expressed it with Kv4.3 and Kv1.4, two channels responsible for fast-inactivating K⁺ currents. Previously, DPP10 has been shown to effect Kv4 channels. However, Kv1.4, when expressed with DPP10, showed many of the same effects as Kv4.3, such as faster time to peak current, and negative shifts in the V of steady-state activation and inactivation The exception was recovery from inactivation, which is slowed by DPP10. DPP10 expressed with Kv4.3 caused negative shifts in both steady-state activation and inactivation of Kv4.3, but no significant shifts were detected when DPP10 was expressed with Kv4.3+KChIP2b. DPP10 and KChIP2b had different effects on closed-state inactivation. At -60 mV, KChIP2b nearly abolishes closed-state inactivation in Kv4.3, while it developed to a much greater extent in the presence of DPP10. Finally, expression of DPP10 mutant consisting of its transmembrane and cytoplasmic 59 amino acids resulted in effects on Kv4.3 gating that were nearly identical to those of wild-type DPP10. These data show that DPP10 and KChIP2b both modulate Kv4.3 inactivation, but that their primary effects are on different inactivation states. Thus, DPP10 may be a general modulator of voltage-gated K⁺ channel inactivation, and that understanding its mechanism of action will lead to deeper understanding of the inactivation of a broad range of K⁺ channels.