The minK protein induces a slowly activating voltage-dependent potassium current when expressed in Xenopus oocytes. We have used macroscopic minK currents to determine the open channel current-voltage relationship for the channel, and have found that the minK current is inwardly rectifying. The channel passes inward current at least 20-fold more readily than outward current. Both rat and human minK exhibit this property. The rectification of minK is similar to that reported for a slow component of the cardiac delayed rectifier, strengthening the hypothesis that minK is responsible for that current.