The primary pathological change in Parkinson's disease is the destruction of dopamine containing cells in the zona compacta of substantia nigra. The cause of nigral cell death and the underlying mechanism remains elusive. However, the discovery of the selective nigral neurotoxin MPTP and its ability to inhibit mitochondrial energy metabolism via its metabolite MPP+ and to generate superoxide radicals suggests processes by which nigral cell death might occur. Recent postmortem evidence in brain tissue from patients dying with Parkinson's disease also suggests the occurrence of some on-going toxic mechanism. This may be a free radical process stimulated by an excess of iron within substantia nigra coupled to a generalised decrease in brain ferritin content. These data suggest altered iron handling occurs in Parkinson's disease which may lead to the generation of toxic oxygen species such as superoxide radicals. There is also evidence for an inhibition of mitochondrial function in the substantia nigra in patients with Parkinson's disease. So there may be a close association between the actions of the synthetic neurotoxin MPTP and the underlying cause of idiopathic Parkinson's disease.