Carbidopa (L-alpha-hydrazino-alpha-methyl-b-(3,4-dihydroxyphenyl) propionic acid is a known inhibitor of aromatic amino acid decarboxylase. In both humans and monkeys, we studied the effects of carbidopa on plasma and brain kinetics of 6-[18F]fluoro-L-DOPA (FDOPA), an analog of L-DOPA used for PET studies of the central dopaminergic system. Pretreatment with carbidopa resulted in increases in the plasma levels of FDOPA and 3-O-methyl-6-[18F]fluoro-L-DOPA (3-OMFD). Total striatal and cerebellar activities measured with PET were also increased. Furthermore, increases observed in the specific striatal activity (striatum minus cerebellum total activity) were correlated with increases in the plasma FDOPA curve. Carbidopa pretreatment did not affect the influx rate constant (K) for FDOPA from plasma to striatum in humans as determined by Patlak graphical analysis. Thus, an increase in measured striatal tomographic activity was secondary to the increase in plasma FDOPA levels rather than as a result of changes in the FDOPA influx rate constant.