The hypothesis is presented, based on the chemical structure of neuromelanin, that one of its functions in the catecholamine neurons in the brain is to protect the cell against toxic quinones (such as dopaminchrome and noradrenochrome, or their dihydroxy isomers) produced from the catecholamines dopamine and noradrenaline (and possibly adrenaline) during the course of prostaglandin synthesis by the enzyme prostaglandin H synthetase, or possibly by spontaneous oxidation. One aminochrome-adrenochrome-has been shown to be neurotoxic and to have psychotomimetic properties in humans. Depending on the site of production these compounds may be involved in the pathogenesis of Parkinson's disease or schizophrenia.