The aim was to investigate the effects of inhibition of monoamine oxidase type B (MAO-B) with selegiline alone and the combined inhibition of peripheral catechol-O-methyltransferase (COMT) with entacapone and MAO-B with selegiline on striatal 6-[18F]fluorodopa (FDOPA) accumulation, and whether the effect of entacapone + selegiline on FDOPA uptake differed depending on the severity of the presynaptic dopaminergic dysfunction. Thus, eight healthy controls, eight de novo patients with Parkinson's disease (PD), and 18 levodopa-treated PD patients were investigated with positron emission tomography (PET). Half of the subjects in each population belonged to the selegiline group and half to the entacapone + selegiline group. Both groups were studied twice with PET using FDOPA. After the first (baseline) FDOPA PET investigation, both groups were on 2 weeks of selegiline treatment, 10 mg daily. Thereafter, the second FDOPA PET was performed for all subjects with a premedication administered 60 min before the PET imaging; one group received 10 mg of selegiline, and the other group received a single 400 mg dose of entacapone coadministered with 10 mg of selegiline. Selegiline treatment alone had no significant influence on striatal FDOPA metabolism. The FDOPA accumulation, expressed as striatal-to-occipital ratios and modified decarboxylation coefficients (k3R0), increased significantly after entacapone + selegiline administration in all subject populations. The FDOPA uptake rate constant (Ki) remained virtually unchanged in controls and in de novo patients but decreased significantly in levodopa-treated PD patients after entacapone + selegiline intake. Entacapone + selegiline administration did not influence significantly the unidirectional blood-to-brain clearance for FDOPA (K1D) or the relative dopadecarboxylase activity (k3D). The changes in the studied parameters after entacapone + selegiline administration probably reflect the effects of entacapone, since entacapone alone has caused similar changes in previous PET studies. Response in FDOPA accumulation to entacapone + selegiline was higher in controls and de novo patients compared with levodopa-treated PD patients. The milder response in levodopa-treated patients might reflect the reduced ability of the degenerated dopaminergic neurons to utilize the prolonged FDOPA availability, produced by entacapone.