The activity of total and heat-stable glutamate dehydrogenase (GDH) in lymphocytes increased with aging in 40 control subjects (p < 0.001). The total GDH activity was lower in patients with olivopontocerebellar atrophy (OPCA), motor neuron disease, Parkinson’s disease or Alzheimer’s disease than in age-matched control subjects, but not in patients with nondegenerative neurological diseases. The decrease in heat-stable GDH activity was observed in patients with OPCA, Parkinson’s disease or Alzheimer’s disease, but heat-labile GDH was decreased only in patients with OPCA.