Using an antibody against choline acetyltransferase (ChAT), mesencephalic cholinergic cell nuclei were studied in autopsy material from 3 cases of progressive supranuclear palsy (PSP) and 4 controls. ChAT-immunoreactive neurons were quantified in sections that spanned the rostrocaudal extent of each nucleus. In PSP, there was a significant decrease in the number of neurons with detectable immunoreactivity for ChAT in and adjacent to the central gray substance in the following nuclei: the nucleus of Edinger-Westphal (69%); the rostral interstitial nucleus of the medial longitudinal fasciculus (97%); the interstitial nucleus of Cajal (78%). A cell loss was also evident in a group of neurons found in the deep layers of the superior colliculus (93%). In contrast, the estimated number of ChAT-immunoreactive cell bodies in cranial nerves III and IV, in the mesencephalic reticular formation, and in the parabigeminal nucleus was not different from that of controls. The results are compatible with the notion that, in PSP, there is a regionally selective destruction of cholinergic neurons.