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The regional brain kinetics following 5-hydroxy-L-(beta-11 C)tryptophan and L-(beta-11 C)DOPA intravenous injection was measured in twelve Rhesus monkeys using positron emission tomography (PET). The radiolabelled compounds were also injected together with various doses of unlabelled 5-hydroxy-L-tryptophan or D-DOPA. The radioactivity accumulated in the striatal region and the rate of increased utilization with time was calculated using a graphical method with back of the brain as a reference region. The rate constants for decarboxylation were 0.0070 +/- 0.0007 (S. D) and 0.0121 +/- 0.0010 min-1 for 5-hydroxy-L-(beta-11 C)tryptophan and L-(beta-11 C)DOPA, respectively. After concomitant injection with unlabelled 5-hydroxy-L-tryptophan, the rate constant of 5-hydroxy-L-(beta-11 C)tryptophan decreased dose-dependently and a 50 percent reduction was seen with a dose of about 4 mg/kg of unlabelled compound. A decreased utilization rate of L-(beta-11 C)DOPA was seen only after simultaneous injection of 30 mg/kg of either L-DOPA or 5-hydroxy-L-tryptophan. This capacity limitation was most likely interpreted as different affinity of the striatal aromatic amino acid decarboxylase for L-DOPA and 5-hydroxy-L-tryptophan, respectively.