Extracellular Zn²⁺ has been identified as an activator of pancreatic K_ATP channels. We further examined the action of Zn²⁺ on recombinant K_ATP channels formed with the inward rectifier K⁺ channel subunit Kir6.2 associated with either the pancreatic/neuronal sulphonylurea receptor 1 (SUR1) subunit or the cardiac SUR2A subunit. Zn²⁺, applied at either the extracellular or intracellular side of the membrane appeared as a potent, reversible activator of K_ATP channels. External Zn²⁺, at micromolar concentrations, activated SUR1/Kir6.2 but induced a small inhibition of SUR2A/Kir6.2 channels. Cytosolic Zn²⁺ dose‐dependently stimulated both SUR1/Kir6.2 and SUR2A/Kir6.2 channels, with half‐maximal effects at 1.8 and 60 μm, respectively, but it did not affect the Kir6.2 subunit expressed alone. These observations point to an action of both external and internal Zn²⁺ on the SUR subunit. Effects of internal Zn²⁺ were not due to Zn²⁺ leaking out, since they were unaffected by the presence of a Zn²⁺ chelator on the external side. Similarly, internal chelators did not affect activation by external Zn²⁺. Therefore, Zn²⁺ is an endogenous K_ATP channel opener being active on both sides of the membrane, with potentially distinct sites of action located on the SUR subunit. These findings uncover a novel regulatory pathway targeting K_ATP channels, and suggest a new role for Zn²⁺ as an intracellular signalling molecule.