The pharmacokinetics of short and long intravenous infusions of levodopa with and without concurrent oral administration of carbidopa was studied in 9 parkinsonian patients. Carbidopa reduced by 50% both the infusion rate required to produce a clinical response and the time required for plasma clearance of levodopa. Using this value for clearance, it is estimated that carbidopa doubles the bioavailability of orally administered levodopa. Carbidopa did not alter the therapeutically effective plasma concentration of levodopa, suggesting that carbidopa does not modify the so-called enzymatic blood-brain barrier. The decline of the plasma levodopa concentration was biphasic; carbidopa modestly increased half-lives of both phases. The apparent volume of distribution was not significantly altered. Carbidopa did not change the duration of the clinical response after the discontinuation of short infusions. From these observations we infer that the therapeutic effects of carbidopa can be attributed to doubling the bioavailability of orally administered levodopa and halving its plasma clearance.