This study was undertaken to investigate the effect of the medial amygdaloid nucleus on the pancreatic exocrine secretion and plasma secretin concentration in 44 male albino rats. Twenty-three rats in which the medial amygdaloid nucleus was damaged bilaterally by radio frequency a.c. through stereotaxically inserted electrodes (medical amygdaloid group, MA) and twenty-one rats which received the same operation without damage (operated control, OC), were prepared. Under urethan anesthesia, 0.01 N hydrochloric acid (HCl) or physiological saline (0.9% NaCl) was infused at a rate of 0.18 ml/min into the duodenum for 20 minutes. Pancreatic jucie was collected for the 20 min infusion period. After collection of pancreatic juice, blood was sampled from the abdominal aorta for the radioimmunoassay of plasma secretin concentration. In the MA group, the exocrine pancreatic secretory response to 0.01 N HCI as well as saline infusion was significantly inhibited compared with that in the OC group. The pancreatic protein output of the MA group significantly decreased after the saline infusion and tended to decrease after the 0.01 N HCI infusion, compared with that of the OC group. However, there was no significant difference in plasma secretin concentration between the two groups. Therefore it is strongly suggested that the rat medial amygdaloid nucleus has a facilitatory influence on both basal and acid-stimulated pancreatic exocrine secretion, but the releasing mechanism of secretin appears not to be involved in the influence.