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A patient with multiple system atrophy developed a regular, rhythmic involuntary movement of the neck muscles which appeared synchronous with respiration and could be described as "rocking-of-the-head". He had been treated with antiparkinsonian drugs such as L-dopa, L-dopa/carbidopa, amantadine hydrochloride and trihexiphenidyl hydrochloride for approximately one year. A fluoroscopic study assured that the involuntary "rocking-of-the-head" movement synchronized with the diaphragmatic up-and-down movement. A polygraphic study showed that the accelerometer curve which reflected the rocking movement of the head oscillated at approximately 0.7 Hz and synchronized with the surface EMG discharges of the right sternocleidomastoid muscle and the nasal flow curve. This involuntary movement was seen almost all day long as well as asleep but severest usually in the afternoon. Discontinuation of trihexiphenidyl hydrochloride alone made the involuntary movement less severe but never suppressed it completely. Resumption of the drug made the involuntary movement as severe as it had been. Discontinuation of L-dopa, L-dopa/carbidopa and amantadine hydrochloride was followed by disappearance of the involuntary movement in a day or so, in spite of continued intake of trihexiphenidyl hydrochloride. Rigidity and bradykinesia induced by discontinuation of these drugs, however, made it necessary to resume them. As a result the involuntary movement again exacerbated.