Inflammatory changes have been found in Parkinson's disease, in humans intoxicated with the parkinsonian toxin MPTP, and in animal models of the disease. However, it is still not known whether inflammatory changes are responsible for active nerve cell death or if they have a protective role against neurodegeneration. In this study, we analyzed the glial reaction in the substantia nigra pars compacta (SNpc) and the striatum of monkeys rendered parkinsosian by chronic MPTP injections. At postmortem examination 1 year after the last MPTP injection, the density of astroglial cells and activated microglial cells in the SNpc, but not in the striatum, of MPTP-intoxicated animals was significantly higher than in the two control animals. These data suggest that neurodegeneration was still active despite the absence of the agent triggering cell death and that the glial reaction is associated with long-term neurodegeneration.