ACTH and some of its fragments have been shown to play a role in a variety of adaptative mechanisms. To clearly identify the nervous structures containing ACTH in the rat brain, an immunohistochemical localization of this peptide was conducted at both light and electron microscopic levels. Nervous fibers staining for ACTH were found to be largely distributed throughout regions of hypothalamus, thalamus, and midbrain. Positive fibers could also be occasionally observed in the spinal cord. Immunostained neuronal cell bodies were only detected in the arcuate nucleus. Essentially, the same results were obtained 2 and 8 weeks after hypophysectomy. In animals pretreated with colchicine, the intensity in the staining of cell bodies was markedly increased, making possible the detection of a larger number of cell bodies. At the electron microscopic level, it was demonstrated that ACTH is contained in dense core vesicles present in nervous fibers and endings. These results indicate that ACTH of nonpituitary origin is synthesized in the central nervous system and could probably be considered as a neurotransmitter of still undefined function.