A dose-response study of the effects of entacapone on the pharmacokinetics and metabolism of levodopa and on the clinical response to levodopa was carried out in 20 parkinsonian patients with levodopa-related fluctuations. A randomized, double-blind, single-graded-dose, crossover design of five 1-day treatment periods each 1 week apart was used. Entacapone (50, 100, 200, or 400 mg) or placebo was given at 8 a.m. with the patient's individual dose of levodopa/dopa decarboxylase inhibitor. The inhibition of soluble catechol-O-methyltransferase (S-COMT) in red blood cells (RBCs) and plasma concentrations of levodopa, its metabolites, and entacapone were measured and motor responses were quantified at 30min intervals using the motor part of the Unified Parkinson's Disease Rating Scale. Entacapone brought about a dose-dependent decrease in S-COMT activity in the RBCs, maximally by 48% at 400 mg. With a 200-mg dose of entacapone, the area under the plasma concentration-time curve (AUC) and half-life of levodopa increased (p < 0.001); the AUCs of 3-O-methyldopa and homovanillic acid decreased (p = 0.01 and p < 0.001, respectively) and that of 3,4-dihydroxyphenylacetic acid increased (p < 0.001). Entacapone prolonged the duration of the motor response to levodopa by 33 min (p = 0.04) and dyskinesias by 45 min (p = 0.003) without affecting their magnitude; the highest increase in duration of these responses occurred with 200 mg of entacapone. Thus, on pharmacokinetic and clinical grounds, the 200-mg dose of entacapone was the most effective. Dose-related responses to entacapone demonstrated its value in the treatment of parkinsonian patients with levodopa-related fluctuations by prolonging the antiparkinsonian response to the levodopa dose.