Most antipsychotic drugs share similar pharmacological properties in that they cause anti-dopaminergic, anti-noradrenergic and anti-serotonergic activities. Using a radioreceptor assay, these anti-aminergic activities were measured in the serum of 34 schizophrenics receiving haloperidol medication. The anti-dopaminergic activity was determined as a value of haloperidol equivalent and chlorpromazine equivalent. The anti-noradrenergic and anti-serotonergic activities were based on the chlorpromazine equivalent. The schizopherenics were divied into two groups (group 1 : 16 out-patients, group 2 : 18 hospitalized acute psychotic patients). Most of the patients in group 1 showed low anti-dopaminergic activity (below 10 ng/ml haloperidol equivalent or 100 ng/ml chlorpromazine equivalent). Anti-noradrenergic and anti-serotoninergic activities could be measured in only two patients. Fifteen patients in group 2 responded well to haloperidol. There was a positive correlation between anti-dopaminergic activity, plasma HPD concentration and daily haloperidol dose in all the group 2 patients. Clinical improvement was positively correlated with anti-noradrenergic activity, but not with anti-dopaminergic activity in the group 2 patients. These findings suggest that anti-noradrenergic activity plays an important role in clinical improvement of schizophrenic patients in an acute psychotic state.