The goal of the study was to determine abnormalities in the spontaneous activity of globus pallidus neurons at the output of the basal ganglia, in cynomolgus monkeys rendered parkinsonian by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In parkinsonian compared to intact monkeys, the mean spontaneous firing rate of the neurons of the internal segment of the globus pallidus (GPi) increased but that of the prevailing neuronal population in the external segment (GPe) inversely decreased. Correspondingly, the mean modal interval between spikes shortened, suggesting increased excitation, in both the GPi and GPe. However, the mean proportion of intervals longer than 100 ms increased in the GPe but remained unchanged in the GPi, suggesting increased inhibition only in the GPe. In the two populations, bursting activities and the mean variability of firing rate increased. Concurrently, a small and distinct neuronal population located in the GPe and another located at the periphery of both the GPi and GPe displayed minor changes, which were however different from those observed in the GPi and in the prevailing neuronal population of the GPe. The intensity of changes varied with time and severity of nigral lesion. In severe parkinsonism, the neuronal activity at the output of the basal ganglia (GPi) is excessive.