While Parkinson's disease has historically been defined in terms of its motor symptomatology, studies have shown that non-motor deficits form an important part of the syndrome. Cognitive deficits can occur even in the early stages of Parkinson's disease. These deficits are often subtle and do not rise to the level of impairment necessary for a diagnosis of dementia; however these deficits are discernable with neuropsychological testing and may produce subjective complaints of cognitive decline and mild functional difficulties in some patients. The traditional pharmacological interventions for Parkinson's disease have focused on controlling and alleviating motor symptoms with levodopa and dopamine agonists. However, these medications treat the symptoms of PD, but do not alter the course or progression of the underlying disorder. In contrast, rasagiline, an MAO-B inhibitor, has recently shown benefits consistent with a possible disease-modifying effect. Given the positive and intriguing findings seen with treatment with rasagiline, the investigators propose to study the effects of this medication on cognition in patients with mild to moderate stage Parkinson's disease.