An outward current of unknown nature increases significantly when cardiac cells are treated with cyanide or subjected to hypoxia^1–4, and decreases on intracellular injection of ATP⁵. 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 (ATP_i) 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.