Alzheimer's disease, Parkinson's disease and Lewy body disease are the most commonly found neurodegenerative disorders in the elderly. The invention is a transgenic mouse that contains two transgenes, human .alpha.-synuclein and human amyloid precursor protein, which are responsible for the formation of the neuritic plaques, Lewy bodies and neurodegeneration seen in the above mentioned diseases. The transgenic mouse is an accurate model for disease by both functional and pathological assays.