The size of neurons in the pars reticulata of the substantia nigra in patients dying with Parkinson's disease has been compared with that in non-parkinsonian control cases. Parkinson's disease is accompanied by a significant (+53%) hypertrophy of reticulata neurons. This is similar to changes seen in experimental rats with ipsilateral damage of the striatum. It is suggested that the enlargement seen in Parkinson's disease, similar to that in the rat, is indicative of plasticity in the GABA-ergic reticulata neurons, and may be associated with increased inhibitory flux in pathways arising from the pars reticulata to the superior colliculus and thalamus.