PD is commonly associated with depression, but conventional antidepressants have limited efficacy in patients with PD and may exacerbate motor symptoms. SAM-e is available in the United States as a food supplement and is promoted as a mood enhancer. SAM-e improves dopamine transmission, may have a beneficial effect on dopamine receptors, and may be a good alternative to the currently-used antidepressants in patients with PD. This study will investigate whether SAM-e is safe and effective in the treatment of depression associated with PD. The efficacy of SAM-e will be compared to placebo and to escitalopram, a selective serotonin reuptake inhibitor commonly used for the treatment of depression in PD.