Numerous peptides have been localized with immunocytochemical techniques in neuronal cell bodies, axons, and nerve endings of the central nervous system (CNS). The widespread distribution of pepndes within the brain and spinal cord implies that these substances may function under physiological conditions as neurotransmitters or neuromodulators. Consequently, a wide variety of techniques for administration to, and recovery from, the CNS for analysis has been utilized to investigate their putative physiological roles.