1. Immunoreactive (IR)-met-enkephalin and beta-endorphin contents in the hypothalamus and the pituitary were measured in alloxan-diabetic rats with or without insulin treatment. 2. Both IR-met-enkephalin and IR-beta-endorphin in the pituitary were substantially reduced in alloxan-diabetic rats 1 month after treatment. 3. Hypothalamic IR-beta-endorphin content was also significantly lower. 4. Gel-filtration chromatography showed that the peaks co-eluting with met-enkephalin precursor, met-enkephalin and beta-endorphin were lower in the pituitaries from the diabetic rats, whereas the peaks co-eluting with beta-endorphin precursor and beta-lipotropin were not. 5. In another experiment, the IR-beta-endorphin contents of the neuro-intermediate lobe and hypothalamus, but not the anterior lobe were significantly lowered in diabetic rats, whereas IR-met-enkephalin contents were significantly reduced in both the anterior and neuro-intermediate lobe. 6. All these changes were reversed by insulin treatment. 7. As a decrease in general protein synthesis could not explain the recorded changes, these results suggest a possible direct role of insulin in regulating the opioid peptide content of the hypothalamus and pituitary.