15 hospitalized patients suffering from neuroleptic-induced tardive dyskinesias participated in a double-blind cross-over study of lithium sulphate and placebo. Each drug was given for 3 weeks. The results were evaluated by means of the video-tape technique. Lithium sulphate induced a slight, but significant reduction in the tardive dyskinesia. Lithium had, in addition, a suppressive effect on psychomotor agitation and aggression. The concentrations of homovanillic acid and 5-hydroxyindoleacetic acid were determined in the cerebro-spinal fluid from five patients. It is concluded that lithium can be used with advantage in the treatment of tardive dyskinesia with only moderate intensity, particularly when the movement disturbances are accompanied by psychomotor restlessness. Finally, the effect of lithium on the central aminergic transmitter substances is discussed in relation to the existing hypotheses on the pathophysiology of tardive dyskinesia.