Frozen-section autoradiography in rat brain sections as well as in vivo positron emission tomography (PET) studies in monkey brain were used for the determination of binding characteristics of O-[methyl-11C]harmine in an attempt to validate this ligand for the assessment of monoamine oxidase A (MAO-A). In frozen sections, the binding of [11C]harmine showed an apparent KD of the binding of 2 nM. The specific binding was inhibited by nanomolar concentrations of clorgyline, esuprone, brofaromine, and Ro 41-1049. The in vivo kinetic pattern in the monkey brain indicated a significant trapping, which was inhibited by pretreatment with clorgyline, moclobemide, or harmine. Different approaches for a quantitative determination of MAO-A enzyme binding were attempted and demonstrated an IC50 dose of harmine in the range of 0.05-0.1 mg/kg. The studies give strong indications for the validity of [11C]harmine as an in vivo tracer for the assessment of MAO-A enzyme binding in the brain.