Colonic mucosa and adenocarcinoma are known to possess gastrin receptors. Recent studies have suggested that some patients with large intestinal cancers and polyps have elevated serum gastrin levels and that gastrin may stimulate growth of colonic neoplasms. The aim of the present investigation was to determine whether endogenous hypergastrinemia--induced by the proton pump inhibitor omeprazole--would influence growth in a subcutaneously implanted murine colonic cancer. The results show that despite a fivefold increase in serum gastrin levels (193 pg/ml median value, range 186-252, in the omeprazole-treated group vs 36 pg/ml median value, range 28-37 in controls), there were no differences in tumor size or survival of tumor-bearing animals. Additionally, there were no differences in serum gastrin values between tumor- (29 pg/ml, range 25-38) and non-tumor- (34 pg/ml, range 25-30) bearing, untreated animals. Endogenous elevation of the serum gastrin hormone to five times the normal level does not demonstrate trophic effects on the murine colon tumor MC-26.