Subjects participating in Project 2 (20 PD subjects on dopaminergic replacement therapy) will also undergo a second [18F]MPPF and PET imaging session, identical except that PD medications will be withheld for approximately 8 hours prior to the imaging session. The second session will occur greater than 7 days, but not more than 3 months, from the first imaging session. Initial 'on' medication quantitative outcomes for each brain region will be compared to the outcomes from the second 'off' medication imaging session to determine the influence of dopaminergic treatment on 5HT1A activity.