A neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which produces pathological changes similar to human idiopathic Parkinson's disease in animals, was injected in mice for biochemical and pharmacological studies. The dopamine concentration showed a marked decrease (-75%) in the striatum 1-2 weeks after the injection of MPTP (30 mg/kg i.p. twice a day for 5 days; a total dose, 300 mg/kg) but no changes or only slight decreases in other brain regions. The norepinephrine concentration also decreased to half the preadministration level in the striatum. These changes closely resembled those observed in the brain of parkinsonian patients. An examination for evaluating bradykinesia of MPTP-treated mice (pole test) devised by the author revealed that bradykinesia was alleviated dose-dependently by injection of L-DOPA. These results suggested that MPTP-treated mice are a useful experimental model for the study of parkinsonism, and that the pole test using these animals is of value in the screening of anti-parkinsonian agents.