Women with premenstrual syndrome (PMS) of such severity that they actively had sought medical attention for their symptoms were compared with healthy female students with regard to platelet MAO activity and temperamental correlates by means of the Karolinska Scales of Personality (KSP), scales from the Eysenck Personality Questionnaire (EPQ) and Eysenck's IVE inventory. The women with PMS were divided into two groups; irritability and depression as predominating symptom. No variation in platelet MAO was found during the menstrual cycle, either in patients or in controls. Both PMS groups had significantly lower platelet MAO activity than the controls. There was no difference between the two groups with PMS. Also with regard to personality traits there were considerable differences between the females with PMS and the controls. There were few differences between the two groups of PMS patients. Thus, the patients scored significantly higher as regards somatic anxiety, muscular tension, indirect aggression, verbal aggression and neuroticism and lower as regards socialization than the controls.