A morphometric study of the striatal complex has been performed in adult and aged rats, by means of Butcher's acetylcholinesterase (AChE) staining. The size of neostriatum, and the number of putatively cholinergic AChE-positive perikarya, are significantly reduced in two-year-old rats. Consequently, mean cell density is decreased in the striatal complex by 15.58%; age-dependent loss is more pronounced: (1) in the dorsal and lateral parts of neostriatum, which in adults have a higher neural density; and (2) in the ventromedial part, which in the aged becomes almost void of neurones. It is concluded that depletion of cholinergic perikarya is a specific feature of natural ageing, which affects diffusely the neostriatum, and particularly its ventromedial territories.