Rectification in inward-rectifier K⁺ channels is caused by the binding of intracellular cations to their inner pore. The extreme sharpness of this rectification reflects strong voltage dependence (apparent valence is ∼5) of channel block by long polyamines. To understand the mechanism by which polyamines cause rectification, we examined IRK1 (Kir2.1) block by a series of bis-alkyl-amines (bis-amines) and mono-alkyl-amines (mono-amines) of varying length. The apparent affinity of channel block by both types of alkylamines increases with chain length. Mutation D172N in the second transmembrane segment reduces the channel's affinity significantly for long bis-amines, but only slightly for short ones (or for mono-amines of any length), whereas a double COOH-terminal mutation (E224G and E299S) moderately reduces the affinity for all bis-amines. The apparent valence of channel block increases from ∼2 for short amines to saturate at ∼5 for long bis-amines or at ∼4 for long mono-amines. On the basis of these and other observations, we propose that to block the channel pore one amine group in all alkylamines tested binds near the same internal locus formed by the COOH terminus, while the other amine group of bis-amines, or the alkyl tail of mono-amines, “crawls” toward residue D172 and “pushes” up to 4 or 5 K⁺ ions outwardly across the narrow K⁺ selectivity filter. The strong voltage dependence of channel block therefore reflects the movement of charges carried across the transmembrane electrical field primarily by K⁺ ions, not by the amine molecule itself, as K⁺ ions and the amine blocker displace each other during block and unblock of the pore. This simple displacement model readily accounts for the classical observation that, at a given concentration of intracellular K⁺, rectification is apparently related to the difference between the membrane potential and the equilibrium potential for K⁺ ions rather than to the membrane potential itself.