Nifedipine can block K⁺ currents through Kv1.5 channels in an open-channel manner (32). Replacement of internal and external K⁺ with equimolar Rb⁺ or Cs⁺ reduced the potency of nifedipine block of Kv1.5 from an IC₅₀ of 7.3 µM (K⁺) to 16.0 µM (Rb⁺) and 26.9 µM (Cs⁺). The voltage dependence of block was unaffected, and a single binding site block model was used to describe block for all three ions. By varying ion species at the intra- and extracellular mouth of the channel and by using a nonconducting W472F-Kv1.5 mutant, we demonstrated that block was conditioned by the ion permeating the pore and, to a lesser extent, by the extracellular ion species alone. In Kv1.5, the outer pore mutations R487V and R487Y reduced nifedipine potency close to that of Kv4.2 and other Kv channels with an equivalent valine. Although changing this residue can affect C-type inactivation of Kv channels, the normalized reduction and time course of currents blocked by nifedipine in 5, 135, and 300 mM extracellular K⁺ concentration was the same. Similarly, a mean recovery time constant from nifedipine block of 316 ms was unchanged (332 ms) after 5-s prepulses to allow C-type inactivation. This is consistent with the conclusion that nifedipine block and C-type inactivation in the Kv1.5 channel can coexist but are mediated by distinct mechanisms coordinated by outer pore conformation.