A dose-ranging study of selegiline was performed in patients with Parkinson's disease to determine the minimal dosage of the drug able to inhibit ≥95% of platelet monoamine oxidase (MAO) activity. Different doses of selegiline (5 or 10 mg daily, 10 or 20 mg weekly) were studied in four groups of six patients with Parkinson's disease. Platelet MAO activity was measured before and after 1 month's treatment with selegiline. The doses of 5 or 10 mg daily and 20 mg (i.e., 10 mg × 2) weekly induced a complete inhibition of platelet MAO-B activity from day 7 to day 28 (96.0–99.5%). In contrast, platelet MAO-B inhibition was only 75.9% of the basal value after a dosage of 10 mg weekly. These results demonstrate that 20 mg weekly is the minimal dosage of selegiline able to induce a maximal and long-lasting inhibition of platelet MAO-B activity in patients with parkinsonism. Further clinical trials are needed to investigate the clinical efficacy of this dose.