Folate deficiency sensitizes mice to dopaminergic neurodegeneration and motor dysfunction caused by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Additional experiments indicate that this effect of folate deficiency may be mediated by homocysteine. These findings suggest that folate deficiency and hyperhomocysteinemia are risk factors for Parkinson's disease.