Mice were injected three times a day for 12 days with 300 micrograms/kg body weight of gastrin G17 or 37.5 Ivy dog U/kg body weight of CCK or saline. Other mice were also injected four times an hr for 1 hr with 7.5 micrograms/kg of gastrin, nine Ivy dog U/kg of CCK or saline; 1 hr before killing, they were injected with tritiated thymidine to evaluate the labelling indices in peptic, antral, duodenal, jejunal, and ileal mucosae. Four hours after the first injection of the two peptides, the peptic labelling indices increased while those of intestinal mucosa increased 8 hr after these injections. Long-term injections of CCK had a trophic effect on secretory cells of the digestive tract: the number of gastric zymogenic cells, Paneth cells, and the mucous cells of Brünner glands were hypertrophied. The pepsin, amylase, chymotrypsin, and lysozyme activities increased in stomach, exocrine pancreas, and intestine, respectively. Neither parietal cells nor intestinal enterocytes and hydrolase activities were affected. The trophic effect of long-term injections of gastrin is confirmed on parietal cells and exocrine pancreatic parenchyma and is demonstrated in Paneth cells. Confirming cytological results, pancreatic lipase and amylase activities and intestinal lysozyme activity were increased after gastrin. Although CCK and gastrin have a structural analogy, these two peptides did not affect the same cellular types. A specific action of CCK on the main secretory cells of the digestive mucosa is demonstrated.