Helicobacter pylori infection is a causal factor of gastric cancer (which is associated with low gastric acid secretion) or duodenal ulcer (high acid secretion). Parietal cells and ECL cells in the stomach are controlled by gastrin, which plays a crucial role in the regulation of acid secretion. The present study was undertaken to identify a possible role of gastrin in determining the different responses of the parietal cells and ECL cells to chronic H. pylori infection. Wild-type (C57BL/6J) gastrin^+/+ mice and gastrin^-/- knockout mice, generated through targeted gene disruption and backcrossed eight times to C57BL/6J, were infected with H. pylori for 9 months. The acid output was measured 4 h after pylorus ligation (known to cause vagal excitation). The gastric mucosa was examined by immunocytochemistry with antisera to α-subunit of H⁺/K ⁺-ATPase for the parietal cells, and to histamine and vesicle monoamine transporter-2 for the ECL cells, and by quantitative electron microscopy. In infected gastrin^+/+ mice, the acid output and the percentage of secreting parietal cells (freely fed state) were 20-30% of the values in uninfected controls, while the density and ultrastructure of parietal cells were normal. The infected mice had hypergastrinemia and displayed hypertrophy and hyperplasia of ECL cells. Although uninfected gastrin ^-/- mice had lower the acid output than uninfected gastrin ^+/+ mice, there was a higher acid output (∼3 times) in infected gastrin^-/- mice than their uninfected homologues. The numbers of parietal cells and ECL cells remained unchanged in infected gastrin ^-/- mice. In conclusion, chronic H. pylori infection results to impaired parietal-cell function (acid hyposecretion), hypergastrinemia and hyperplasia of ECL cells in wild-type mice but leads to vagally induced hypersecretion in gastrin-deficient mice.