Three groups of schizophrenic patients were treated with haloperidol, with a low dose of piribedil (a dopamine agonist), and with a combination of the two treatments, respectively. After a few days, all 7 patients treated with the drug combination showed marked rigidity and akinesia, while patients treated with haloperidol alone (4) and piribedil alone (4) showed either mild or no symptoms of parkinsonism. The drug combination induced mainly an akinetic-hypertonic syndrome, while tremors were absent or mild. The results suggest that low doses of the DA-agonist potentiate the extrapyramidal side effects of haloperidol by acting on self-inhibitory DA receptors, thereby blocking the compensatory increase in dopaminergic firing elicited by the neuroleptic agent.