Dimethylphenethylamine (DMPA), a substrate for the B-form of the monoamine oxidase enzyme (MAO, EC 1.4.3.4), was labelled with 11C in two different positions; in the methyl group (M-DMPA) and in the phenethyl group (P-DMPA). M-DMPA was prepared by N-alkylation of methylphenethylamine with [11C]methyl iodide and P-DMPA was prepared by N-alkylation of dimethylamine with [1-11C]phenethyl iodide. The radiochemical yields were 30-35% (M-DMPA) and 10% (P-DMPA), based on [11C]carbon dioxide, with overall synthesis times of 30-35 min (M-DMPA) and 50 min (P-DMPA). The compounds were isolated by semi-preparative HPLC and the radiochemical purity was in both cases greater than 99%. The biodistributions of 11C-labelled M-DMPA [correction of M-DPMA] and P-DMPA were studied in rat brain by dissection and in the brain of a Rhesus monkey by positron emission tomography (PET).