Tachykinins exert a broad range of actions in the mammalian nervous system. While much is known about the localization of peptides derived from one of the two mammalian tachykinin genes (substance P- and neurokinin A-encoding preprotachykinin), little has been reported on the localization of peptides derived from a second tachykinin gene encoding neurokinin B. Using an antiserum raised against a 30-residue peptide fragment (Peptide 2) of the protein precursor to neurokinin B, we have mapped the distribution of Peptide 2 by immunocytochemistry. Peptide 2 antiserum specificity was determined by western blot analysis (which showed antibody cross-reactivity to a neurokinin B fusion protein from a cloned neurokinin B-encoding complementary DNA) and by the elimination of immunoreactive product in brain tissue sections upon preabsorption with a 10 microM concentration of Peptide 2 peptide. In addition, we report on the distribution of neurokinin B-messenger RNA with a full-length complementary RNA probe to localize cells that express the neurokinin B precursor. Peptide 2 immunoreactivity and neurokinin B-messenger RNA-positive cells were found, in some instances, paralleling the distribution of substance P and in other cases existing separately from substance P. Peptide 2 immunoreactivity as well as neurokinin B-messenger RNA-positive cells were found in the main olfactory bulb, cortex, olfactory tubercle, nucleus accumbens, hippocampus, bed nucleus of the stria terminalis, amygdala, medial habenula, periaqueductal gray, superior and inferior colliculus, and nucleus of the spinal trigeminal tract. Whereas substance P is found throughout the rat brain, neurokinin B appears to be partitioned more to forebrain than to brainstem structures. The marked differences in the distribution of both tachykinins in the rat central nervous system suggests that neurokinin B may play an important role in olfactory, gustatory, visceral, and neuroendocrine processing of information.