A fundamental issue in molecular pharmacology is to define how agonist-receptor interaction differs from that of antagonist-receptor interaction. The V(1a) vasopressin receptor (V(1a)R) is a member of a family of related G-protein-coupled receptors (GPCRs) that are activated by vasopressin, oxytocin (OT) and related peptides. A segment of the N-terminus that was required for agonist binding, but not antagonist binding, was identified by characterizing truncated V(1a)R constructs. Site-directed mutagenesis revealed that a single residue (Arg(46)) was critical for agonist binding and receptor activation. The N-terminus of the related OT receptor (OTR) could recover agonist binding in a chimaeric OTR(N)-V(1a)R construct. Furthermore, Arg(34) of the human OTR, which corresponds to Arg(46) of the rat V(1a)R, provided agonist-specific binding epitopes in the OTR, indicating a conserved function of this locus throughout this GPCR subfamily. Mutation of Arg(46) revealed that high-affinity agonist binding had an absolute requirement for arginine at this position.