Acid and pepsin secretion and gastric mucosal histidine decarboxylase activity were measured in rats of various ages between 5 and 40 days after injection of saline, pentagastrin, histamine, or carbachol. Basal acid secretion first appeared on day 15. At this time carbachol significantly stimulated both acid and pepsin secretion. Gastrin and histamine did not stimulate acid or pepsin secretion until day 20. Histidine decarboxylase activity first appeared on day 10 and was first increased by pentagastrin on day 18. Injection of 8-day-old rats with corticosterone prematurely induced acid secretion on day 12 in response to all three stimulants and pepsin secretion in response to carbachol only. These studies provide a comprehensive picture of the development of gastric mucosal sensitivity to the three naturally occurring stimulants and indicate that adrenal glucocorticoids play an important role in that development.