To investigate the development of pancreatic exocrine function and intestinal negative feedback regulation with aging in rats, we measured pancreas weight, content of amylase and trypsinogen in the rat pancreas and plasma CCK concentrations, activity of amylase and trypsin in the small intestine at an hour after oral administration of trypsin inhibitor (TI), and also examined amylase secretory response to CCK-8 in the rat pancreatic acini at various ages in vitro. As a result, amylase content per pancreas weight increased with the age and amylase activity in the small intestine at al ages showed a significant increase in TI group compared to controls. Plasma CCK concentrations were elevated after administration of TI at all ages. Amylase release from pancreatic acini stimulated by CCK-8 responded poorly on days 7, then gradually increased with age, showing a biphasic dose response curve with maximal response of 10(-10) M of CCK-8 from 14-day-old to 66-day-old. The results indicated that the mechanism of pancreatic secretory response to TI already might exist at the stage of sucking rat and secretory response to CCK-8 in vitro showed a low response, and developed with age.