The cross-sectional area of cholinergic neurons stained immunohistochemically for choline acetyltransferase were measured in the striatum of infants, young adults, aged individuals and patients with Alzheimer's disease. Cholinergic neurons were distributed throughout the caudate nucleus and the putamen and displayed no significant variations in size across the two structures. The cross-sectional area of striatal cholinergic neurons was smaller in infants as compared with all other groups examined. No significant changes were observed in the size of these neurons as a consequence of aging or Alzheimer's disease.