Mutational analysis of the tachykinin NK-1 (refs 1-7), NK-2 (ref. 8) and angiotensin AT-1 (refs 9, 10) receptors indicates that non-peptide antagonists act through residues located between the seven transmembrane segments, whereas natural peptide agonists bind mainly to residues scattered in the exterior part of the receptor. The presumed contact points for the prototype NK-1 antagonist CP96,345 cluster on opposing faces of the outer portions of transmembrane helices V and VI (refs 1-5). Here we show that systematic introduction of histidyl residues at this antagonist-binding site in the human NK-1 receptor gradually converts it into a high-affinity metal-ion-binding site without affecting agonist binding. In a double mutant with histidine residues substituted at the top of transmembrane segments V and VI, respectively, Zn2+ inhibits binding of radiolabelled agonist peptide and efficiently blocks phosphoinositol turnover induced by substance P. We propose that Zn2+ and CP96,345 act as 'allosteric competitive' antagonists by stabilizing inactive conformations of the mutant and the wild-type receptor respectively. Introduction of metal-ion-binding sites could be used as a general tool in the structural and functional characterization of helix-helix interactions in G-protein-coupled receptors, as well as in other membrane proteins.