The distribution of histidine decarboxylase-like immunoreactivity (HDCI) in the rat central nervous system was studied by the indirect immunofluorescence technique. HDCI cell bodies were concentrated in the posterior hypothalamic area, such as in the tuberal magnocellular nucleus, caudal magnocellular nucleus, posterior hypothalamic nucleus and lateral hypothalamus just lateral to the fasciculus mammillothalamicus at the level of the posterior hypothalamic nucleus. Extensive networks of HDCI fibers of various densities were found in many areas of the brain; they were particularly dense in the hypothalamus but were also found in the following areas: rostrally in the cerebral cortex, olfactory nuclei, medial amygdaloid nucleus, n. tractus diagonalis, and bed nucleus of the stria terminalis, and caudally in the central gray matter of the midbrain and pons, auditory system, n. vestibularis medialis, n. originis nervi facialis, n. parabrachialis, n. commissuralis, n. tractus solitarii, and n. raphe dorsalis.