The distribution of neuromedin Us, uterus-stimulating and hypertensive peptides newly identified in porcine spinal cord, was examined in the rat brain by the indirect immunofluorescent method. Neuromedin U-like immunoreactive structures were found to be unevenly distributed in the neuronal system. Neuromedin U-like immunoreactive neurons were present in the cranial motor nuclei, reticular nuclei, nucleus vestibularis lateralis, trigeminal sensory nuclei, colliculus superior and inferior, lemniscus lateralis, nucleus pontis, nucleus ruber, zona incerta, substantia innominata, horizontal limb of the diagonal band and cerebral cortex. The immunoreactive fibres were found in the above areas, particularly near the labelled cells, forming a fibre plexus with various intensities of immunoreactivity. In addition, dense plexuses were also seen in the nucleus reticularis thalami, nucleus ventralis posteromedialis, nucleus ventralis posterolateralis, nucleus tegmentalis dorsalis and ventralis, vertical limb of the diagonal band, nucleus olivaris superior, and nucleus pontis. In the first six structures, no labelled neurons were present and in the remaining structures, a few scattered neurons were noted. This indicates that these fibres are probably of extrinsic origin.