High quality PACAP-related peptide (PRP), a 29 amino-acid region of the PACAP precursor protein, has been synthesized in quantities sufficient for biological and structural studies. PRP has a distinct biological activity on the gallbladder that is similar to PACAP, but opposite to that of VIP and its related peptide, PHM. Its solution structure has been investigated by circular dichroism spectroscopy and 2D 1H nuclear magnetic resonance spectroscopy. In contrast to the poorly defined structure in aqueous solution alone, the limiting structure, under conditions that mimic a membrane-like environment, possesses stable secondary structure with a helical region between residues 3 and 20, that is terminated by the presence of glycine at residue 21 and is followed by a region of nascent helix. The similarities and differences in the structure of PRP, PACAP27 and GHRH(1-29) are made through comparison of their H alpha chemical shift data and differences in their biological activities assessed.