The effect of short-term infusion of a large dose of pentagastrin and a small dose of synthetic human gastrin I (SHG) on the rate of cell proliferation in gastric mucosa was studied in normal human subjects. Moreover, the kinetic parameters were compared with the serum gastrin concentrations in fasting patients. Endoscopic biopsies were labelled in vitro with 3H-thymidine and autoradiographs were prepared. The percentage of DNA-synthesising cells in the progenitor cell region was estimated. In healthy volunteers infusion of a large dose of pentagastrin (10 mug/kg per hour) was followed by a marked increase in the labelling index in fundic mucosa. The antral mucosa was not responsive to this effect. In the same subjects, infusion of a low dose of SHG (8 ng/kg per min) did not affect the rate of cell proliferation, either in fundic or in antral mucosa. In 46 patients with different gastric diseases no correlation between the serum gastrin concentrations and the labelling indices was found. The results suggest that human fundic mucosa is responsive to a trophic action of pentagastrin. If it exists, however, a physiological action of gastrin as a trophic hormone for human gastric mucosa must be considerably more complex than previously believed.