The motor and cognitive effects of a selective D-1 dopamine receptor agonist, SKF 39393, were assessed in patients with Huntington's disease, Gilles de la Tourette's syndrome, tardive dyskinesia, and torsion dystonia, using a double-blind placebo-controlled design. Over daily doses ranging from 3.2 to 32 mg/kg and treatment intervals extending from one to seven weeks, no consistent changes could be discerned. The contribution of D-1 receptor mediated mechanisms to the pathophysiology of hyperkinetic extrapyramidal disorders remains uncertain.