Scrapie is a degenerative disease of the central nervous system of sheep and goats. The causative agent has been passaged to a number of laboratory species, including mice and hamster. Amyloid plaque formation and vacuolation, the signs of senile dementia, are found in the brains of mice infected with 87V scrapie agent. Dopamine (DA) and norepinephrine (NE) concentrations in the brains of scrapie-infected mice were measured with high-performance liquid chromatography-electrochemical detector (HPLC-ECD). A significant decrease in NE level was exhibited in all regions tested, whereas the level of DA decreased significantly only in cerebral cortex. Immunohistochemistry was used to examine immunoreactive catecholamine neurons in substantia nigra and locus ceruleus using antisera against tyrosine hydroxylase (TH). The population of TH-immunoreactive neurons in the substantia nigra and locus ceruleus were significantly decreased in scrapie-infected mice compared to controls. These data suggest that both the noradrenergic and dopaminergic system are sensitive to the action of scrapie agent 87V and that changes in the catecholamine levels in the brains of scrapie-infected mice may contribute to some of the clinical symptoms of the diseases, such as ataxia and apraxia.