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 K1 channels and acetylcholine-regulated K+ channels from the heart are examples of strong inward rectifiers and ATP-sensitive K+ channels are weak rectifiers(1,7-10). 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.