The cholinergic innervation of the mediodorsal nucleus of the thalamus, which is thought to originate primarily in the laterodorsal tegmental nucleus and the substantia innominata, was studied by acetylcholinesterase histochemistry and immunohistochemistry with a polyclonal antiserum against human choline acetyltransferase on autopsy tissue from eight control subjects, five patients with progressive supranuclear palsy and four patients with senile dementia of Alzheimer type. In controls, cholinergic innervation of the mediodorsal nucleus of the thalamus was distributed heterogeneously in densely labelled patches surrounded by less heavily stained matrix. In patients with progressive supranuclear palsy, the density of choline acetyltransferase-positive varicosities decreased by 75% in the matrix and 60% in the patches. The number of choline acetyltransferase-positive cell bodies decreased by 84% in the laterodorsal tegmental nucleus, but more moderately (-33%) in the substantia innominata. In patients with senile dementia of Alzheimer type, choline acetyltransferase-positive varicosities decreased by 34% in the matrix, but 46% in the patches. Choline acetyltransferase-labelled cell bodies were spared in the laterodorsal tegmental nucleus, whereas severe loss (-80%) was observed in the substantia innominata. These results suggest that cholinergic innervation of mediodorsal nucleus matrix derives mainly from the laterodorsal tegmental nucleus and mediodorsal nucleus patches from the substantia innominata. Differential loss of innervation to the matrix and patches in progressive supranuclear palsy and senile dementia of Alzheimer type may in turn differentially affect mediodorsal nucleus innervation of the frontal cortex, resulting in dissimilar symptomatologies.