The chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model we have developed in monkey reproduces all the cardinal features of Parkinson's disease, and, in particular the characteristic slow evolution of clinical signs. We still know little, however, of the kinetics of the nigral degeneration induced. This present study charts the progressive destruction of tyrosine hydroxylase-immunoreactive neurones in mice treated daily with low doses of MPTP for 20 days. Our results show that the neuronal death rate is initially high, subsequently decreases, and stabilizes. This new protocol thus mirrors closely the pattern of evolution assumed to be that of Parkinson's disease and should prove useful for studies on neuroprotection and compensatory mechanisms.