This paper deals with several new findings regarding the trophic action of gastrointestinal hormones. Naturally occurring gastrins (G-17 I, G-17 II, G-34 II) stimulated DNA synthesis and increased total DNA content of gastric mucosa. These were several times more potent than pentagastrin on a molar basis. In a survey of various tissues from the gastrointestinal tract, pentagastrin exerted trophic effects on mucosa of the oxyntic gland area, duodenum, and colon; it had no effect on the esophagus, antrum, or diaphragm. Maximal stimulation (200% of control) of colonic DNA synthesis was produced by 250 µg of pentagastrin per kg. Vasoactive intestinal peptide did not stimulate DNA synthesis when given alone and inhibited the trophic effect of pentagastrin when administered simultaneously. Glucagon stimulated DNA synthesis in both colon and oxyntic gland mucosa to 40% of the increase caused by pentagastrin and did not inhibit the effects of pentagastrin when administered concurrently.