1. Epidermal growth factor (EGF) infused subcutaneously in a dose of 10 micrograms/kg . h but not 1 microgram/kg . h inhibited spontaneous gastric acid and pepsin secretion, whereas when given intragastrically in a dose of 10 micrograms/kg . h it failed to affect this secretion. 2. EGF injected intraperitoneally at 8 h intervals for 24 h significantly stimulated DNA synthesis in the gastroduodenal mucosa and the pancreas, whereas when administered intragastrically it stimulated DNA synthesis only in the gastroduodenal mucosa but not in the pancreas. 3. Chronic parenteral administration of EGF significantly increased the DNA and RNA contents of the gastroduodenal mucosa and the pancreas. 4. This study demonstrates that parenteral EGF is a potent inhibitor of gastric secretion and trophic agent for the gastroduodenal mucosa and pancreas, and that the gastric inhibitory and trophic effects of EGF are the results of two separate mechanisms.