The effects of streptozotocin-induced diabetes on pituitary neuropeptides were studied. Substance P, dynorphin and β-endorphin in both pituitary lobes and cholecystokinin and somatostatin in the neurointermediate lobe () were measured 4 weeks after streptozotocin treatment in adult male rats. There were significant decreases of substance P levels in both the anterior lobe (AL) and , and of cholecystokinin, dynorphin and β-endorphin in the , whereas the dynorphin content in the AL increased, when values were expressed on a per-lobe basis. On a per-milligram-protein basis, however, only β-endorphin in the showed a significant decrease, while AL β-endorphin and dynorphin were increased. Correlated with these changes were a drastic decrease in the serum insulin level and a marked increase in serum glucose and corticosterone levels. All these changes were reversible with insulin treatment. It is suggested that the decrease in contents of neuropeptides demonstrated (except for β-endorphin) might be due to mechanisms other than a change in synthesis.