The relationship between antipsychotic effect and side effects and plasma levels of clozapine were investigated in 26 patients. A significant linear correlation was found between dosage and plasma levels of clozapine. All patients were clearly improved. The degree of improvement was independent of the plasma levels in an average range of plasma levels (100 ng/ml-800 ng/ml). Sedation and orthostatic dysregulation occurred most pronounced on the third day of treatment. There was no relationship between plasma levels and these symptoms. After prolonged treatment the degree of both symptoms diminished. The development of tolerance of symptoms is concluded.