The distribution of neuropeptide K (NPK), a 36-residue amidated peptide originally isolated from porcine brain, is described in the rat CNS by immunohistochemical methods. Antibodies were generated in rabbits to N-terminus and C-terminus regions of the peptide and the distribution of immunoreactive cell bodies and fibers was mapped in colchicine-treated and normal rat brains. Major areas of cell body staining included the medial habenular nucleus, the ventromedial nucleus of the hypothalamus, the interpeduncular nucleus, the lateral dorsal tegmental nucleus, the nucleus raphe pallidus, and the nucleus of the solitary tract. Some of the areas of dense NPK-fiber immunoreactivity included the ventral pallidum, the caudate-putamen, certain areas of the hypothalamus, the central and medial amygdaloid nuclei, the entopeduncular nucleus, the habenular nuclei, the substantia nigra pars reticulata, the caudal part of the spinal nucleus of the trigeminal nerve, the nucleus of the solitary tract and the dorsal horn of the spinal cord. A striking similarity exists between this pattern of immunoreactive staining and that described for substance P, suggesting that the tachykinin systems do not exist independently in the brain. The possible roles for multiple tachykinins in the brain are discussed.