- The authors examined the advantage of addition of the adenylate cyclase inhibitor demeclocycline (DMC) versus placebo to haloperidol therapy in the treatment of excited psychosis. Sixteen manic or excited schizoaffective patients completed a double‐blind controlled five week trial. Nine patients were assigned to DMC and haloperidol and seven patients were assigned to placebo and haloperidol. Brief Psychiatric Rating Scale (BPRS) was used as an assessment tool at baseline, 1, 2, 3, 4 and 5 weeks. BPRS scores showed no significant difference between the two study groups. However, the reduction in BPRS score was 23·5 units in the DMC group and 16·7 units in the placebo group; power analysis showed that this trend would be significant if the size of the study were doubled. Adenylate cyclase inhibitors might have a therapeutic role in the treatment of excited psychosis. © 1998 John Wiley & Sons, Ltd.