Major depression is present, at any given time, in 20-40% of Parkinson's disease (PD) patients, several times the prevalence in the general population. In addition, depression may precede the diagnosis of PD. These observations and reports of depression during deep brain stimulation of regions contiguous to the substantia nigra, as well as reports of dopamine agonist improving depression, suggest depression, rather than being mainly a psychological reaction to a debilitating disease, is part of PD. It is postulated that mesolimbic and mesocortical dopaminergic pathways that mediate affect, behavior, and cognition, contribute to depression in PD.