Levels of methionine-enkephalin immunoreactivity (MEI) and beta-endorphin immunoreactivity (BEI) were measured by means of a specific RIA in pituitary, hypothalamus, pancreas, and adrenal glands of db/db and control mice. We found significantly higher levels of MEI in the pituitaries of db/db mice than in either littermate or background strain controls. There was no significant difference in MEI levels in pancreas, adrenal glands, and hypothalamus. There was a significant difference in the BEI levels in the pituitaries of db/db mice compared with those in controls. No significant difference in BEI levels was observed between db/db and control mice in any of the other regions examined. We conclude from the above data that the opiate peptide system in the hypophyseal-hypothalamic axis of the db/db mouse is abnormal and warrants further investigation. The significance of this finding with respect to the possible etiology of diabetes mellitus is discussed.