We investigated the transport of levodopa from blood to brain in control and in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) parkinsonian monkeys by using intracerebral microdialysis. The degree of parkinsonism was established by the monkey's clinical score and its postmortem striatal dopamine level. We administered levodopa plus carbidopa either orally or by intravenous injection. We estimated the blood-to-brain transport of levodopa as the ratio of its concentration in the brain's extracellular fluid to that in arterial plasma. This ratio was reduced in the MPTP parkinsonian monkeys, and we found an inverse relationship between the monkeys' ability to transport levodopa from blood to brain and their degree of parkinsonism. The oral administration of a high-protein meal or the intravenous infusion of large neutral amino acids before the administration of the levodopa plus carbidopa reduced the transport of levodopa into the brain, as measured by microdialysate collected from the striatum. We found that in two animals an intraperitoneal injection of the beta-adrenergic agonist isoproterenol increased the blood-to-brain transport of levodopa. Pharmacologic manipulation of the transport of levodopa from blood to brain may offer a new strategy for the treatment of Parkinson's disease.