 This is a pilot evaluation of dopaminergic function in PD using a bolus plus constant infusion protocol with 123I-IBZM and SPECT to evaluate the potential for carbidopal-dopa alone or carbidopal-dopaentacapone to produce displacement of striatal radioactivity for assessment of intrasynaptic dopamine. We will assess the feasibility of this paradigm for detecting l-dopa effects on the SPECT signal in subjects with PD with disease duration of greater than 4yrs and with a stable response to L-dopa. Each subject will undergo three 123I-IBZM studies separated by 1-2 weeks. Subjects will be off medication for at least 12 h prior to study For each of the three scan days patients will receive a constant intravenous infusion of 123I-IBZM over 4-5 hours to establish an equilibrium binding condition of the radiotracer at striatal D2D3 receptors. Three baseline SPECT acquisitions will be obtained prior to medication dosing to establish a stable baseline. At approximately 5 h after the initiation of the infusion subjects will receive a single oral dose of either carbidopalevodopa 37.5mg150mg or 50mg250mg, or carbidopalevodopaentacapone either 37.5mg150mg200mg- STALEVO or 50250mg200mg. 