There is evidence to suggest that glutamate and other excitatory amino acids play an important role in the regulation of neuronal excitation. Glutamate receptor stimulation leads to a non-physiological increase of intracellular free Ca2+. Disturbed Ca2+ homeostasis and subsequent radical formation may be decisive factors in the pathogenesis of neurodegenerative diseases. Decreased glutamatergic activity appears to contribute to paranoid hallucinatory psychosis in schizophrenia and pharmacotoxic psychosis in Parkinson's disease. It has been suggested that a loss of glutamatergic function causes dopaminergic over-activity. Imbalances of glutamatergic and dopaminergic systems in different brain regions may result in anti-akinetic effects or the occurrence of psychosis. The simplified hypothesis of a glutamatergic-dopaminergic (im)-balance may lead to a better understanding of motor behaviour and psychosis.