Six psychotic patients were included in a four-week study of the effects of the D1 selective antagonist SCH 39166 given as monotherapy. Four had a diagnosis of schizophrenia, and two suffered from a schizoaffective disorder. All presented with an acute psychotic exacerbation at the beginning of the trial. SCH 39166 was progressively increased from 50 mg/day to 600 mg/day. In the four schizophrenic patients, the BPRS worsened, and three out of the four failed to complete the study because of this. Three schizophrenic patients were aggressive or violent after abrupt discontinuation of treatment. In the two patients with schizoaffective disorder the BPRS improved during the trial, but they had an acute relapse immediately after treatment discontinuation. Extrapyramidal symptoms improved in three of the six patients, and worsened in one.