As a metabolite of arginine-vasopressin, AVP(4-8) has been shown to have potent memory-enhancing activity and to induce a series of physiological and biochemical events in rat brain. GTP-binding protein is known to be a revolving stage of transmembrane signal transduction to mediate physiochemical responses of neurotransmitters and neuromodulators. A specific binding site of AVP(4-8) in the rat hippocampal synaptic membranes was identified by radio-receptor assay and after binding to membranes, AVP(4-8) enhanced the binding of Guanosine -5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS), and this enhancement could be completely reversed by the antagonist of AVP(4-8), ZNC(C)PR. Based on the alone results, we suggest that AVP(4-8) exerts its function as neurotransmitter through a G-protein-coupled receptor on the synaptosomal membrane of rat hippocampus.