Mastomys is a rodent with a high incidence of spontaneous carcinoids in the acid-producing part of the stomach. The present study was conducted to examine whether hypergastrinemia could promote tumor formation in this species. Mastomys, 4 months of age, were treated for 5 months with omeprazole subcutaneously, 100 mumol/kg body weight daily, and compared with mastomys given the vehicle only. The plasma gastrin concentration and the number of antral gastrin cells were increased in the omeprazole-treated group. The hypergastrinemia was associated with elevated histidine decarboxylase activity and histamine content in the oxyntic mucosa and with a trophic effect on the oxyntic mucosa and the enterochromaffin-like cells. However, no carcinoid tumors were observed, possibly because the strain of mastomys studied does not produce carcinoids spontaneously.