The effects of intracellular pH on an inwardly rectifying K+ channel ("Kin channel") in opossum kidney (OK) cells were examined using the patch-clamp technique. Experiments with inside-out patches were first carried out in Mg2(+)- and adenosine triphosphate (ATP)-free conditions, where Mg2(+)-induced inactivation and ATP-induced reactivation of Kin channels were suppressed. When the bath (cytoplasmic side) pH was decreased from 7.3 to either 6.8 or 6.3, Kin channels were markedly inhibited. The effect of acid pH was not fully reversible. When the bath pH was increased from 7.3 to 7.8, 8.3 or 8.8, the channels were activated reversibly. The channel activity exhibited a sigmoidal pH dependence with a maximum sensitivity at pH 7.5. Inside-out experiments were also carried out with a solution containing 3 mM Mg-ATP and a similar pH sensitivity was observed. However, in contrast with the results obtained in the absence of Mg2+ and ATP, the effect of acid pH was fully reversible. Experiments with cell-attached patches demonstrated that changes in intracellular pH, which were induced by changing extracellular pH in the presence of an H+ ionophore, could influence the channel activity reversibly. It is concluded that the activity of Kin channels can be controlled by the intracellular pH under physiological conditions.