Vasoactive intestinal polypeptide (VIP) is a gut neuroendocrine polypeptide that increases cyclic adenosine monophosphate (cAMP) production in cells with VIP receptors. Some gastrointestinal cancer cells possess functional receptors for VIP; however, the role of VIP in regulation of growth of gastric cancer cells has not been determined. The purpose of this study was to determine whether VIP and other agents that increase cAMP regulate growth of a human gastric cancer cell line (AGS) and whether these agents regulate expression of c-myc proto-oncogene, which is required for cell proliferation. We measured levels of cAMP by radioimmunoassay, and we used Northern blot analysis to examine c-myc messenger RNA expression. Cell-growth studies were carried out in media supplemented with 3% serum, and cells were counted with a Coulter counter. We found that VIP significantly increased cAMP production of AGS cells in a dose-dependent manner, whereas secretin, glucagon, and peptide histidine methionine (PHM) did not stimulate cAMP production. Exogenous cAMP (8-bromo-cAMP) inhibited AGS cell growth in a dose-dependent manner. VIP acted synergistically with either isobutylmethyl-xanthine or forskolin to inhibit AGS cell proliferation. The increased c-myc expression, which was induced by serum, was inhibited by simultaneous treatment with VIP and isobutylmethyl-xanthine. We have found that AGS cells have specific, functional VIP receptors (activation of which are negatively correlated with cell growth) and that the mechanism by which VIP acts to inhibit cell growth appears to be due, in part, to cAMP-dependent regulation of c-myc proto-oncogene expression.