Trimipramine and clozapine show some similarities in their receptor binding profiles. Since both have the same affinity for the D2 receptor and since the affinity for this receptor correlates closely with the antipsychotic potency of a drug, an antipsychotic efficacy of trimipramine in acute schizophrenia could be expected. Therefore 28 schizophrenic patients in an acute phase were treated with trimipramine up to 400 mg/d in an open clinical trial. For the whole group of patients the BPRS total score changed from 58 +/- 5 before treatment to 46 +/- 18 at the last rating (p less than 0.05). According to our clinical judgement the patients were divided into three subgroups. Thirteen patients showed a good remission under trimipramine so that they could be discharged on a trimipramine maintenance treatment. They improved on the BPRS from 58 +/- 6 before treatment to 32 +/- 8 at endpoint. Six patients deteriorated during the first week of treatment and had to be withdrawn from the study. Nine patients showed insufficient improvement or became worse after an initial improvement. The observed side-effects (dry mouth, sedation, sweating, increased appetite, constipation, tremor, vertigo) are well known under trimipramine and were therefore expected. Beyond these, one patient developed a cardiac insufficiency. No clinical relevant extrapyramidal side-effects occurred. Since the improvement of florid psychotic symptoms seems to be markedly higher under trimipramine than the one reported under placebo, our results indicate that trimipramine may have an antipsychotic potency.