The inwardly rectifying K channel provides the resting K conductance in a variety of cells. This channel acts as a valve or diode, permitting entry of K+ under hyperpolarization, but not its exit under depolarization. This behaviour, termed inward rectification, permits long depolarizing responses which are of physiological significance for the pumping function of the heart and for fertilization of egg cells. Little is known about the outward currents through the inwardly rectifying K channel, despite their great physiological importance, and the mechanism of inward rectification itself is unknown. We have used improved patch clamp techniques to control the intracellular media, and have recorded the outward whole-cell and single-channel currents. We report here that the channel conductance is ohmic and that the well-known inward rectification of the resting K conductance is caused by rapid closure of the channel accompanied by a voltage-dependent block by intracellular Mg2+ ions at physiological concentrations.