Activity levels of platelet monoamine oxidase (MAO), plasma amine oxidase (PAO), erythrocyte catechol-o-methyltransferase, and dopamine-beta-hydroxylase were measured in 24 drug-free patients with Tourette's syndrome (TS) and in 24 normal control subjects matched for age and sex. Only MAO and PAO activity levels were significantly higher in patients than in controls. Activity of the four enzymes was uncorrelated with age, sex, age of onset of illness, severity of illness, family history of tics and Ts, and ethnic-religious background. The hypothesis that probands with heavily loaded pedigrees or with Ashkenazi ethnic-religious background would be more biochemically deviant was not supported.