The effects of pH on K⁢ currents were investigated in single smooth muscle cells isolated from the thoracic aorta of Wistar-Kyoto rats. Whole-cell K⁢ currents were recorded in the conventional configuration of the voltage-clamp technique. Pinacidil (10uM) activated the whole-cell current and the pinacidil-activated current was completely inhibited by glibenclamide (10uM) , an inhibitor of ATP-sensitive K⁢ channel (KATP channel). Pinacidil-activated current was reversed at near the K⁢ equilibrium potential. This current was time- and voltage-independent and reduced by elevating intracellular ATP. Pinacidil-activated current was reduced by lowering the external pH. However, alteration of internal pH has controversial effects on pinacidil-activated current. When the single cell was dialyzed with 0.1 mM ATP, alteration of internal pH had no effect on pinacidil-activated K⁢ current. In the contrast, when the single cell was dialyzed with 3 mM ATP, pinacidil-activated current was increased by lowering internal pH. Our results suggest that K⁢ channel activated by pinacidil may be KATP channel and internal H⁢ may reduce the inhibitory effect of ATP on KATP channel.