Several animal and human epidemiologic studies have provided evidence that exercise may be neuroprotective in Parkinson's disease (PD). Exercise may forestall diagnosis and, in the case of those who have already been diagnosed with PD, it may slow the observed neurodegeneration. Unfortunately, because this line of research is in early stages, there is little evidence to indicate what biological mechanisms underlie the neuroprotection that is conferred with exercise. Toward this end, it is possible that an interaction between endogenous antioxidant enzymes, inflammatory processes, and reactive oxygen species may be associated with exercise improvements in PD.