Mg²⁺, Na⁺, and TEA⁺ block Kir1.1 channels from the inside with similar voltage dependence. Currents through Kir1.1 channels were measured in inside-out patches with 11 mM K⁺ plus 99 mM Na⁺ in the extracellular (pipette) solution and 110 mM K⁺ in the cytoplasmic (bath) solution, with or without 1 mM Mg²⁺, 3 mM TEA⁺, or 20 mM Na⁺. (A) Typical current traces at voltages of −100 mV and +40 mV. Dashed lines show the closed state of the channel. (B) i-V relationships. Data represent means ± SEM for 3–10 different patches. (C) Analysis of voltage dependence. Fractional block is linearized and fit to Eq. 3. Linear regression gives estimates of K_i(0) = 1.6 (Mg²⁺), 1.8 (TEA⁺), and 160 (Na⁺) mM, and zδ = 0.58 (Mg²⁺), 0.61 (TEA⁺), and 0.72 (Na⁺).