An outward current of unknown nature increases significantly when cardiac cells are treated with cyanide or subjected to hypoxia, and decreases on intracellular injection of ATP. We report here that application of the patch-clamp technique to CN-treated mammalian heart cells reveals specific K+ channels which are depressed by intracellular ATP (ATPi) at levels greater than 1 mM. For these channels, conductance in the outward direction is much larger than the inward rectifier K+ channel which is insensitive to ATP. AMP had no effect on the ATP-sensitive K+ channel, and ADP was less effective than ATP. Thus, the ATP-sensitive K+ channel seems to be important for regulation of cellular energy metabolism in the control of membrane excitability.