INWARD-RECTIFIER potassium channels conduct K⁺across the cell membrane more efficiently in the inward than outward direction. This unusual conduction property is directly related to the biological action of these channels^1–6. One basis for inward rectification is voltage-dependent blockade by intracellular Mg²⁺ (refs 1, 7–9): strong inward-rectifier channels are so sensitive to intracellular Mg²⁺ that no outward K⁺ current is measurable under physiological conditions; weak inward rectifiers are less sensitive and allow some K⁺ to flow outwards. Background Kl channels and acetylcholine-regulated K⁺ channels from the heart are examples of strong inward rectifiers and ATP-sensitive K⁺ channels are weak rectifiers^1,7a–10. Here we show that mutations at one position in the second transmembrane segment can alter the Mg²⁺affinity and convert a weakly rectifying channel (ROMK1) into a strong rectifier. The amino acid at this position exposes its side chain to the aqueous pore and affects Mg²⁺ blockade as well as K⁺ conduction through an electrostatic mechanism.