Oxidative stress has been proposed as a pathogenetic mechanism in Parkinson's disease (PD). One mechanism of oxidative cellular injury is the nitration of protein tyrosine residues, mediated by peroxynitrite, a reaction product of nitric oxide and superoxide radicals. We demonstrate here the presence of nitrotyrosine immunoreactivity in Lewy bodies within melanized neurons and in amorphous deposits associated with intact and degenerating neurons. The core of the Lewy body was frequently intensely immunolabeled, while the rim was lightly labeled or unlabeled. This likely reflects the fact that tyrosine residues of neurofilament proteins are primarily localized to Lewy body cores, and suggests that nitrotyrosine is present in neurofilament protein itself. Although these observations are as uet unable to provide a definitive link between oxidative stress and neuronal dysfunction, they demonstrate that oxidative stress has occurred within the vulnerable neurons of PD, leaving a permanent marker of oxidative modification of neuronal proteins within the target cells of neurodegeneration. In addition, these observations provide a potential link between excitotoxicity and oxidative stress within the vulnerable neurons of PD and represent a pathogenetic mechanism in common with the 2 other major age-related nuerodegenerative diseases, Alzheimer disease and amyotrophic lateral sclerosis.