Background and aim: Pituitary adenylate-cyclase activating peptide (PACAP) is a more potent proliferative agent than gastrin for rat enterochromaffin-like (ECL) cell proliferation in vitro. The role of this neurotransmitter during gastrin-mediated ECL cell tumor formation and gastrin-autonomous ECL cell neoplasia is unknown. Methods and results: ECL cell transformation was induced in the Mastomys using 16 wk H 2 receptor blockade of acid inhibition. Examination of the epithelial fundic mucosa demonstrated that PACAP-immunoreactivity significantly increased in the tumor mucosa compared to the nave stomach, and was associated with ECL cells. Nave and tumor ECL cells were then purified (~95%) from Mastomys and the presence of all three PACAP/VPAC receptor subtypes was demonstrated by polymerase chain-reaction amplification. Thereafter, cells were maintained in short-term (48 h) primary cultures. PACAP significantly (p<0.05) increased 24 h bromo-deoxyuridine uptake (~4-fold) in both cell types with estimated EC 5 0 values of ~4x10 - 1 6 M and ~2x10 - 1 6 M, respectively. Specific receptor antagonists (PAC 1 /VPAC 1 ) of PACAP competitively inhibited these proliferative effects in nave cells. Oligonucleotide antisense directed against PAC 1 significantly inhibited PACAP-stimulated DNA synthesis by ~85% (p<0.05) in tumor cells. Conclusion: PACAP is a potent and effective modulator of ECL cell proliferation. The expression of this neuropeptide and its receptors, particularly PAC 1 , suggest the existence of a neural regulatory pathway of ECL cell proliferation and transformation.