In parkinsonian brain, TH and the BP cofactor, as well as DBH and PNMT with phenylethanolamine as substrate, are decreased. However, decrease in AADC with L-DOPA as substrate was not statistically significant. TH activity has also been shown to be decreased in striatonigral degeneration and Shy-Drager syndrome. TH reduction only in striatum but not in substantia nigra was found in a case of juvenile Parkinson's disease. Changes in AADC with L-DOPA or L-5-HTP as substrates varied in parkinsonian brains. PNMT with norepinephrine as substrate was also significantly decreased in parkinsonian brains. After administration of tyrosine to mice, the BP concentration was increased in striatum, adrenals, and serum. Mean BP concentration in serum of parkinsonian patients was slightly but significantly lower than that in controls. The serum BP increased three- to sevenfold in response to an oral tyrosine load in controls, whereas there was no increase or only a minor one (less than threefold) in parkinsonian patients. Our present results indicate that all catecholamine-synthesizing enzymes and BP synthesis may be impaired in Parkinson's disease.