A common form of Parkinsonism today is that induced by certain ataractic drugs, particularly phenothiazines containing a piperazine moiety. Mild to severe extrapyramidal symptoms can appear and disappear within hours or days of commencing or withdrawing the drugs. With rare exceptions the syndrome is completely reversible. This study was based on the postulate that the syndrome is due to interference with the normal functioning of central catecholamines, particularly dopamine, and of central histamine. Extrapyramidal reactions caused by reserpine or 1 of 3 phenothiazines were treated with from 4 to 32 gm. per day of dihydroxyphenylalanine (dopa, the precursor of the catecholamines), with 0.4 to 0.6 gm. per day of diphenhydramine (an antihistamine), or with each in turn. Dopa was mildly beneficial in 4 out of 22 patients, whereas diphenhydramine was completely effective in each of 11