Acepromazine was tried in 48 chronic mental patients, 12 male and 36 female. Their ages ranged from 22 to 61 years, and the time elapsed from their first admission in a mental hospital ranged from 1 to 29 years.The majority of the patients were suffering from various types of schizophrenic reactions, and no significant number of other diagnostic categories was included.Acepromazine was administered orally for an average of 16 weeks. The highest daily dosage reached 120 mg., with an average of 99.58 mg. The drug did improve symptoms and behavior in a significant manner.Thirty-seven out of the 48 patients showed some improvement; 7 showed no change; 1 patient worsened. The other 3 patients worsened and developed toxic reactions; the medication was discontinued and the patients were removed from the study.There was a slight indication that younger patients and those admitted recently were helped more than older patients or those whose first admission was many years ago. There was no significant difference in the effect of acepromazine on various diagnostic categories. There was no relation between the total length of the medication and the degree of improvement. There was a slight but not significant indication that a slower increment in dosage results in greater improvement than a faster increment. Two main areas, Psychotic Symptoms and Behavioral Traits, improved more than Intellectual Resources, Character Traits and Interpersonal Relationships, and Socialization and Rehabilitation, but the improvement in Behavioral Traits is not significantly correlated with the improvement in the Psychotic Symptoms.With the exception of 3 patients for whom the drug had to be discontinued because of toxic reaction characterized by extreme agitation, confusion and weakness, very few side effects have occurred and none was serious. No jaundice or allergic reactions were noted, and a very low incidence of Parkinsonism was encountered. No relation was found between the number or the type of the side effects, including Parkinsonism, and the degree of improvement.Finally, it was noted that in this series of patients, acepromazine proved effective where other drugs or EST were not and that of the 7 patients who showed no improvement on acepromazine, none had improved on previous chemotherapy or EST.