Positron emission tomography (PET) was applied to investigate the rate of turnover of pig brain MAO-B. For this purpose 11C-L-deprenyl was used as an irreversible ligand, which bind stoichiometrically to the enzyme. A tracer dose of 11C-L-deprenyl was injected and PET scans performed to obtain baseline deprenyl binding. A high dose of unlabelled deprenyl was then administered to inhibit the enzyme and tracer doses of 11C-L-deprenyl, with subsequent PET analyses, were given at 0, 2, 7, 21 and 42 days. The half-life for the turnover rate calculated was found to be 6.5 days.