Levodopa is the most efficacious drug in the symptomatic treatment of Parkinson's disease. However, exogenously administered levodopa is extensively metabolized in the periphery by aromatic amino acid decarboxylase (AAAD) and catechol-O-methyltransferase (COMT) so that only 1% of an administered dose gains access to the brain. Even when levodopa is co-administered with an inhibitor of AAAD such as benserazide or carbidopa, the bulk (90%) of levodopa is converted by COMT to the therapeutically inactive 3-O-methyldopa. Two COMT inhibitors, tolcapone and entacapone, have recently been introduced as adjuncts to levodopa to further inhibit peripheral levodopa metabolism and thereby enhance brain levodopa availability. This paper reviews the pharmacokinetics, dosing schedule, peripheral and central effects, and safety profile of these agents.