Plasma dopamine-beta-hydroxylase activity (DBH) was estimated in 50 drug-free Parkinson patients, 30 of whom had never been treated, in 26 patients treated chronically with L-Dopa, and in 44 controls. 22 of the drug-free patients were examined again after two weeks treatment with L-Dopa plus decarboxylase inhibitor. In disagreement with other reports, no differences were found among the groups and no change in the enzyme activity occurred after treatment, although urinary homovanillic acid increased 20 fold. Low DBH activities could not be attributed to the age of onset or duration of illness, main symptom, depression, autonomic disturbances, or duration of drug treatment. The only difference was found in patients with dementia associated to their illness, who showed low enzyme activities.