Current evidence indicates that symptoms of Parkinson's disease are related to depletion of dopamine in the corpus striatum. Levodopa, the metabolic precursor of dopamine, does cross the blood-brain barrier, and presumably is converted to dopamine in the brain. Carbidopa inhibits decarboxylation of peripheral levodopa. Since its decarboxylase inhibiting activity is limited to extracerebral tissues, administration of carbidopa with levodopa makes more levodopa available for transport to the brain.