The tachykinin family of neuropeptides, which includes substance P, neurokinin A, and neurokinin B, have three distinct receptors: NK-1, NK-2, and NK-3. With the cloning of the rat NK-3 cDNA, it is now possible to evaluate the distribution of NK-3 mRNA in the rat brain. Female rat brains were sectioned and hybridized with a riboprobe complimentary to NK-3 mRNA. The results of these studies revealed an extensive distribution of NK-3 mRNA throughout the rostral-caudal extent of the brain, spinal cord, and retina. In agreement with previous binding studies, we observed NK-3 mRNA in the cortex, the amygdala, the hippocampus, the medial habenula, the zona incerta, the paraventricular and supraoptic nuclei of the hypothalamus, the substantia nigra, the ventral tegmental area, the interpeduncular nucleus, the raphe nuclei, the dorsal tegmental nucleus, and the nucleus of the solitary tract. In contrast with binding data, only a few NK-3 mRNA cells were detected in the striatum. In addition, the present study detected NK-3 mRNA in the olfactory bulb, the dentate gyrus and subiculum, the medial septum, the diagonal band of Broca, the ventral pallidum, the globus pallidus, the bed nucleus of the stria terminalis, the arcuate, the premammillary and mammillary nuclei, the dorsal and lateral regions of the posterior hypothalamus, the central gray, the cerebellum, the parabrachial nuclei, the nucleus of the spinal trigeminal tract, the dorsal horn of the spinal cord, and the retina. The results of these in situ hybridization histochemical studies have provided detailed and novel information about the distribution of NK-3 mRNA and have elucidated the putative sites of neurokinin B action in the rat central nervous system.