The rostal mesencephalon at the level of the posterior commissure was studied by light microscopy in two patients with idiopathic Parkinson's disease, one patient with Alzheimer's disease, and one patient with senile dementia of Alzheimer's type. In the Parkinsonian cases, the rostral part of the nucleus of Edinger Westphal disclosed Lewy bodies in 3% of the neurons, neurofibrillary degeneration in 2% of the neurons, and a 54% neuronal loss. In Alzheimer's disease, 2% of Edinger Westphal neurons contained neurofibrillary degeneration, whereas in senile dementia of Alzheimer's type only rare neurofibrillary degeneration was evident in this nucleus. Neuronal loss was not apparent in the nucleus of Edinger Westphal in either of the Alzheimer's cases. The pathologic changes observed in this presumably cholinergic nucleus resemble in some respects changes reported in the cholinergic centers of the basal forebrain in these diseases. In addition, the central gray matter and pretectal region in Parkinson's disease contained a patchy increase in astroglia, some with scant reactive cell bodies; however, Lewy bodies were limited to that part of the central gray matter corresponding to the nucleus of Darkschewitsch. A few neurofibrillary tangles were present in the nucleus of Darkschewitsch in both diseases.