The ionic properties underlying the inwardly rectifying potassium current in cultured voltage-clamped white bass horizontal cells were studied. Anomalous rectification was apparent upon membrane hyperpolarization with a reversal potential depolarized from the predicted value of EK. In raised extracellular potassium, the current increased and the reversal potential shifted toward a more depolarized membrane potential. Solutions containing decreased sodium caused a rapid decrease in the inward rectifier current but only slightly affected the reversal potential. Extracellular cesium or barium caused a reversible voltage-dependent reduction of the inward current. We interpret these results to mean that the inward rectifying channel in white bass horizontal cells is mainly permeable to potassium ions, but is sodium dependent. It may shape the photoresponses of the horizontal cells and may contribute to a hyperpolarization activated conductance increase measured in situ.