The effects of porcine gastrin and histamine on deoxyribonucleic acid synthesis and on mitotic activity in the fundic glands were explored in dogs. Fundic mucosal specimens were labeled in vitro with H³-thymidine and autoradiographs were prepared. Twelve hours after administration of gastrin, at doses corresponding to a maximal acid secretion, the labeling index in the mucosa was abruptly increased, whereas mitotic index rose only from the 20th hr. Such a sequence of kinetic features clearly indicated that cell proliferation was stimulated. By contrast, no significant variations of the proliferative parameters were observed after maximal stimulation of acid secretion with histamine. This confirms the hypothesis that gastrin exerts a trophic action on the fundic mucosa.