Markers of dopaminergic synaptic activity and choline acetyltransferase (CAT) were measured in the putamen and caudate nucleus of a patient who lived 36 hours after a unilateral mechanical lesion of the mesencephalon. After cessation of impulse flow along the nigrostriatal tract, dopamine was elevated, dihydroxyphenylacetic acid was diminished, and CAT and tyrosine hydroxylase activity were enhanced in the putamen ipsilateral to the lesion. [3H]-spiperone binding indicated an increase in D2-dopamine receptor density in the caudate nucleus. These findings indicate that the changes predicted from experimental neurochemical models occur in human nigrostriatal systems.