The distribution of neuropeptide Y (NPY)-like immunoreactive (NPYI) structures in the rat lower brainstem was examined by means of indirect immunofluorescence or peroxidase-antiperoxidase methods. In addition to the well known immunoreactive NPYI containing cell groups, the present study demonstrated a much wider distribution of immunoreactive NPYI cells in the lower brainstem, i.e. substantia nigra pars lateralis, interpeduncular nucleus, inferior colliculus, nucleus cuneiformis, dorsal tegmental nucleus of Gudden, nucleus laterodorsalis tegmenti, nucleus vestibularis medialis, nucleus vestibularis inferioris, in the fasciculus longitudinalis medialis and nucleus parvocellularis compacta. We also demonstrated an extensive network of NPYI fibers in various areas of the lower brainstem including the auditory system, viscerosensory system, visceromotor system, raphe nuclei, reticular formation, parabrachial area, locus coeruleus and interpeduncular nucleus, etc. The possible importance of NPY is briefly discussed.