In the substantia nigra, acetylcholinesterase may have a non-cholinergic function. Previous work suggests that release of acetylcholinesterase locally in the substantia nigra leads to a net increase in dopaminergic activity in the ipsilateral striatum. To investigate this hypothesis, acetylcholinesterase was microinjected bilaterally into the substantiae nigrae of awake rats and stereotyped behaviour used as an indication of increased dopaminergic activity in the striatum. Acetylcholinesterase increased stereotypy in rats, while butyrylcholinesterase and the vehicle, distilled water, were ineffective. The functional significance of acetylcholinesterase in the substantiae nigrae of freely moving animals is discussed in the light of its apparent association with dopaminergic rather than cholinergic systems.