Tertiapin (TPN), a small protein derived from honey bee venom, inhibits the GIRK1/4 and ROMK1 channels with nanomolar affinities. Methionine residue 13 in TPN interacts with residue F148 in the channel, located just outside of the narrow region of the ROMK1 pore. The methionine residue in TPN can be oxidized by air, which significantly hinders TPN binding to the channels. To overcome the reduction in TPN affinity due to oxidation of M13, we replaced M13 in TPN with fourteen different residues. Out of the fourteen derivatives, only the one in which M13 was replaced by glutamine, TPNQ, binds to the channel with a Ki value very similar to that of native TPN. Since TPNQ is stable and functionally resembles native TPN, it will be a very useful molecular probe for studying the inward-rectifier K+ channels.