Mesoridazine besylate (Serentil) was evaluated in an eight-week open-label trial in 12 male and 41 female patients with chronic schizophrenia who had ceased to progress beyond a certain level of functioning despite continued psychotropic medication not including Serentil. After a ten-day washout period the patients received Serentil in I.M. doses of 25-150 mg daily on the first ten study days, followed by daily tablet doses ranging from 25 to 400 mg until the final day of the study. Observations were made before and after the washout period as well as on days 3, 10, 14, 28 and 56 of the study, and results were expressed in terms of the Nurses' Observation Scale for Inpatient Evaluation (NOSIE) and the Brief Psychiatric Rating Scale (BPRS). The NOSIE ratings showed that the patients were significantly "less mentally ill" at the end of the washout period and underwent a statistically highly significant further improvement in the course of the Serentil treatment. These improvements occurred in irritability, manifest psychosis, retardation, and depressive manifestations, and social competence. Only two BPRS items improved significantly during the washout period. Treatment with Serentil led to improvement in thinking disturbances as well as in psychomotor, paranoid and depressive disturbances. In fact, between baseline and day 56 of the treatment period, all BPRs items except guilt feelings and all BPRS factors were improved to a highly significant (p less than 0.01) degree. Rating for mental illness from study day 10 onward showed that patients in a subgroup of depressive disturbances showed significantly greater improvement than patients in another subgroup, psychomotor disturbances. Adverse reactions were experienced by a total of 13 patients; in 7 of these a dosage adjustment became necessary for this reason. All patients completed the study program.