Parkinson's disease (PD) is a common neurodegenerative disorder affecting approximately 80,000 Veterans, representing a priority area for VA research. Current medicines for PD only improve symptoms, treatments that slow disease progression are needed, and earlier diagnosis of PD may be the key to their development. PD symptoms can be mimicked by medicines (most commonly antipsychotic drugs that block dopamine), and some of these patients actually have underlying "prodromal" PD that was "unmasked" years before it would have caused symptoms. This problem is increasing as these medicines are now used for common conditions including post-traumatic stress disorder and depression. The investigators will identify prodromal PD in patients with drug-induced symptoms using brain scans. These patients will be enrolled in a randomized clinical trial of aerobic exercise which slows progression in animal models of PD and has other health benefits. The investigators will measure the effect of exercise on symptoms, disease progression (using brain scans) and markers of PD risk (using blood tests). These studies will improve early PD diagnosis and potentially identify a way to slow progression of PD.