• The actions of haloperidol, dextroamphetamine sulfate, levamfetamine succinate, apomorphine, and piribedil were studied in two patients with Gilles de la Tourette's disease in an attempt to clarify the catecholamine mechanisms involved in this condition. Both dextroamphetamine and levamfetamine increased the severity of the symptoms; dextroamphetamine was more potent. Haloperidol controlled the symptoms and also antagonized the effect of dextroamphetamine. Apomorphine injections reduced the severity of symptoms, even in the presence of dextroamphetamine. We conclude that dopamine rather than norepinephrine is the principal catecholamine responsible for the symptoms. The effect of apomorphine may be understood through its action on postulated presynaptic inhibitory dopamine receptors, or other presynaptic mechanisms of action.