L-Dopa is metabolized to 3–O-methyldopa (3OMD) by catechol-O- methyltransferase (COMT). This reduces the amount of L-dopa available for entry into brain. We studied the effect of OR-611, a new COMT inhibitor, on plasma and brain 6-[18F]-fluoro-L-dopa (6FD) metabolism in cynomolgus monkeys with positron emission tomography (PET). OR-611 pretreatment substantially reduced plasma 6FD metabolism to 3-O-methylfluorodopa (3OMFD). PET measurements of striatal 6FD concentrations showed an average 2.3-fold increase following OR-611 pretreatment, compared to the same animals in the control state. OR-611 inhibits plasma metabolism of 6FD and increases brain uptake of this L-dopa analog. OR-611 appears to be a promising agent as an adjunct to L-dopa for the treatment of patients with Parkinson's disease.