24 hospitalized psychiatric patients with neuroleptic-induced tardive dyskinesia were treated with alpha-methyl-para-tyrosine (AMPT), 4 g daily for 3 days, and biperiden, 12 mg daily for 3 weeks. The results were evaluated blindly by means of videotape technique. The frequency of tardive dyskinesia was significantly reduced by AMPT and significantly increased by biperiden. The amplitude was reduced by AMPT in ten cases, unchanged in 12 cases and increased in two cases. Biperiden significantly increased the amplitude. The duration of each separate tongue protrusion and/or mouth opening was significantly increased by AMPT and reduced or unchanged by biperiden. It is concluded that a reduced dopaminergic activity (pharmacological or organic) may constitute the primary pathogenetic background for tardive dyskinesia, but that dopaminergic hypersensitivity and/or cholinergic hypofunction is necessary before the hyperkinetic element of the movement disturbances can minifest itself.