Gastrin was given to rats by continuous subcutaneous infusion through implanted osmotic minipumps in doses covering a wide range of the dose response relationship for gastrin with regard to the trophic effect on the enterochromaffin like cells of the oxyntic mucosa. Thirty five rats were divided into five groups (each of seven rats), one group receiving a control solution of 1% albumin, the others receiving gastrin in 1% albumin at doses of 2.5, 5, 10, and 15 micrograms/kg/h, respectively. The plasma gastrin concentrations in the various groups increased in the same order of magnitude as expected from the gastrin doses given. Gastrin induced a dose dependent increase in enterochromaffin like cell density, oxyntic mucosal histamine concentration and histidine decarboxylase activity up to the dose of 5 micrograms/kg/h, where the increase levelled off. Hence, the dose response relationship for the trophic effect of gastrin on the enterochromaffin like cells seems to follow a polynomial rather than a linear function. These findings may also contribute to the understanding of the trophic effect of gastrin on enterochromaffin like cells in man with conditions associated with hypergastrinaemia.