We measured metabolites of tyrosine and tryptophan (TRP) in the frontal cortex, putamen (PT), and pars compacta of the substantia nigra (SN) of control and Parkinson's disease (PD) brain tissues. Dopamine concentrations were significantly decreased in the PT and SN of PD tissue, regardless of L-dopa therapy. However, 3-O-methyldopa (3OMD) concentration showed a significant increase in each region of the PD group treated with L-dopa (PD[+]) as compared with both the control group and the PD group without L-dopa therapy (PD[-]). Therefore, 3OMD concentration appears to be a reliable marker of L-dopa therapy. Serotonin concentration was lower in each region of the PD groups than in the control group. Although the magnitude of decrease was greater in the PD(+) group, there was no statistical significance between the two PD groups. The same patterns of decrease were present in kynurenine (KYN) and kynurenic acid (KYA) concentrations, but the molar ratios of TRP to KYN and KYN to KYA were unchanged among three groups. In contrast, 3-hydroxykynurenine (3OHKY) concentration was increased in the PT and SN of both PD(-) and PD(+) groups, and the ratio of KYN to 3OHKY was significantly reduced in the PT of the PD(-) group and in three regions of the PD(+) group. Since the KYN pathway leads to formation of nicotinamide-adenine dinucleotide (NADH), the present results may be a further indication of a defect in NADH:ubiquinone oxidoreductase (complex I) in mitochondria in PD. © 1992 American Academy of Neurology.