The peroxidase-antiperoxidase immunohistochemical method has been used in order to investigate the distribution of neurons immunoreactive with antiserum against neuropeptide Y in the hedgehog and the sheep brain. Neuropeptide Y-immunoreactive bipolar and multipolar neurons, with a diameter ranging from 10-30 microns, were found in the following regions: cerebral cortex, striatum, Ammon's horn, gyrus dentatus, amygdala, anterior olfactory nucleus, nucleus accumbens, area along the medial forebrain bundle, arcuate nucleus of the hypothalamus and lateral reticular nucleus of both hedgehog and sheep brain. Furthermore, while neuropeptide Y-immunoreactive neurons were also found in the medial and lateral septal nucleus, interstitial nucleus of the stria terminalis, retrochiasmatic area, central gray substance of the midbrain, colliculus caudalis, and nucleus of the solitary tract of the sheep, respective regions of the hedgehog brain appeared to be devoid of immunoreactive perikarya. This distribution pattern is identical to that observed with antiserum against avian pancreatic polypeptide. The differences in the distribution of neuropeptide Y-immunoreactive neurons in the two species studied as well as between our results and those in other species are briefly discussed.