Gastrin stimulates the growth of gastric mucosa by increasing mostly its glandular region but is not known to induce the growth of a pit region where its major constituent cells, gastric surface mucous (GSM) cells, turn over rapidly. To investigate the effect of gastrin on GSM cells, we generated hypergastrinemic mice by expressing a human gastrin transgene. We obtained a hypergastrinemic mouse line whose average serum gastrin level is 671 ± 252 pg/ml (normal level <150 pg/ml). Gastrin-positive cells were found in the fundic mucosa. The gastric mucosa exhibited hypertrophic growth, which was characterized by an elongated pit with an active proliferative zone, but the glandular region containing parietal cells was normal or reduced in size. The GSM cells contained fewer mucous granules than those of control littermates and lost reactivity to the GSM cell-specific cholera toxin β-subunit lectin. GSM cells along the foveolar region and many mucous neck cells became Alcian blue positive, suggesting the appearance of sialomucin in these cells. We suggest that gastrin stimulates the growth of the proliferative zone of gastric glands, which results in the elongation of the pit region whose GSM cells exhibit less-differentiated features.