Receptors for the neurotransmitter and neuroendocrine peptides, vasoactive intesinal peptide (VIP) and secretin, both belong to the Type B subfamily of G-protein-coupled receptors. This group is evolutionally as well as structurally distinct from the much larger Type A, or rhodopsin-type, subfamily. We have mapped the ligand-activating epitopes of the human VIP1 receptor by the use of hybrid receptor constructs with the human secretin receptor. Twelve chimeras were synthesized the successively replacing portions of the former receptor with corresponding portions of the latter receptor, or by interchanging the first extracellular loops. Each of the different chimeric receptor DNAs were then expressed in murine reporter cells, and their ability to activate cAMP production was investigated on stimulation with the respective natural peptide ligands. We stimulated the reporter cells with secretion or VIP following transient expression of the receptor chimeras. The experiments indicated that there are two molecular domains of importance for the recognition and activation of these peptides, namely, the inner portion of the extracellular tail and the first extracellular loop of the two receptors.