We studied the microglial reaction in mice using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced model for Parkinson's disease (PD). Microglial cells were identified by means of the Griffonia simplicifolia lectin (GSA-I-B4). Dopaminergic neurons were marked by tyrosine hydroxylase antibodies. Microglial activation was demonstrated by an increase in cellular number and changes of morphology (increased lectin staining, larger cell bodies and thicker processes) were seen in the substantia nigra from the 1st to the 14th day and in the striatum from the 1st to the 4th day after intoxication. Depletion of dopaminergic neurons was most pronounced 7 and 14 days following the treatment. The results suggest that microglial activation may be involved in the sequence of pathological changes that lead to dopaminergic neuronal damage after MPTP intoxication.