Tolcapone is a mixed (peripheral and central) catechol-O-methyltransferase (COMT) inhibitor, whereas entacapone is a preferential peripheral COMT inhibitor. Both drugs are able to decrease the peripheral conversion of L-DOPA into 3-O-methyl-DOPA and thereby increase plasma and cerebral levels of L-DOPA, the precursor of dopamine (DA). Tolcapone may also impair the extraneuronal catabolism of DA by inhibiting COMT activity in the brain. To evaluate the role played by peripheral and central COMT inhibition, we compared the effects of tolcapone and entacapone on COMT activity in peripheral tissues, and on striatal extracellular levels of L-DOPA and DA in rats. Tolcapone and entacapone, at the dose of 15 mg/kg p.o., were almost equally effective in inhibiting COMT activity in duodenum and liver. Tolcapone decreased striatal extracellular levels of homovanillic acid (HVA), thus confirming its central COMT inhibitory effect, whereas entacapone did not alter HVA efflux. Following L-DOPA/benserazide administration (50/15 mg/kg p.o.), both COMT inhibitors significantly increased striatal levels of L-DOPA and DA compared with saline. The levels of L-DOPA were similar after treatment with either COMT inhibitors, whereas the increase in DA output was significantly greater in rats given tolcapone compared to those given entacapone. We conclude that the blockade of central DA catabolism by tolcapone contributes to the greater increase in striatal DA levels achieved with this drug.