Six monkeys treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine developed a Parkinsonian syndrome (rigidity, akinesia, flexed posture and tremor). In both high and low dose groups, neurons in the substantia nigra were selectively damaged. At high dose levels, nigral neurons were severely damaged, but because the monkeys died, the evolution of the pathology could not be studied. At low dose levels, some nigral neurons survived, and a significant number of these nerve cells showed reductions in the immunoreactivity of tyrosine hydroxylase. Axonal pathology was conspicuous in the nigrostriatal pathway. Loss of the immunoreactivity of tyrosine hydroxylase in perikarya may represent a retrograde axonal reaction, a potentially reversible response. The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model should prove useful for investigating abnormalities occurring as a consequence of dysfunction of the nigrostriatal system, for examining processes associated with repair of damaged neuronal systems, and for developing and testing therapeutic approaches designed to prevent or ameliorate the Parkinsonian syndrome.