Beta-endorphin (60-91), and beta-lipotropin are present in various tissues but are mostly concentrated in the pituitary, particularly in the pars intermedia. In brain, beta-endorphin is highly localized in the hypothalamus; however, it cannot be detected in striatum, cortex or cerebellum (0.01 nmole/mg protein). In contrast, the highest concentration of enkephalin is found in the stratum. This dissimilar distribution of enkephalin and beta-endorphin strongly suggests that beta-endorphin and enkephalin are stored in separate neurons and probably the beta-endorphin immunoreactive material of various molecular weights present in the pituitary and hypothalamus suggests that in both tissues, beta-endorphin is derived from beta-lipotropin which, in turn, is formed from a larger molecular weight precursor presumably 31K beta-endorphin.