It is known that glucose-induced depolarization^1–4 of pancreatic B-cells is due to reduced membrane K⁺-permeability^5–8 and is coupled to an increase in the rate of glycolysis⁹, but there has been no direct evidence linking specific metabolic processes or products to the closing of membrane K⁺ channels. During patch–clamp studies of proton inhibition of Ca²⁺-activated K⁺ channels [G_K(Ca)] in B-cells¹⁰, we identified a second K+-selective channel which is rapidly and reversibly inhibited by ATP applied to the cytoplasmic surface of the membrane. This channel is spontaneously active in excised patches and frequently coexists with G_K(Ca) channels yet is insensitive to membrane potential and to intracellular free Ca²⁺ and pH. Blocking of the channel is ATP-specific and appears not to require metabolism of the ATP. This ATP-sensitive K⁺ channel [G_K(ATP)] may be a link between metabolism and membrane K⁺-permeability in pancreatic B-cells.