Pathophysiological mechanism of irritable bowel syndrome (IBS) was analyzed by experimental psychological stress. The subjects were 30 cases with typical IBS and 20 cases of control. After an electric transducer was inserted in the sigmoid colon for measuring colonic intraluminal pressure, mirror drawing test (MDT) was loaded as psychological stress with examination of various factors of autonomic nervous function and gastrointestinal hormones. Loading with stress interview and autonomic drugs followed it. The results were as follows ; (1) Systolic and diastolic blood pressure and plasma catecholamines elevated significantly during MDT stress in both groups (p<0.05). (2) Colonic motility index and frequency of intraluminal pressure waves showed a significant increase in the IBS group (p<0.01) compared with those of the control group (p<0.001) during MDT stress. (3) Coefficient of variation of RR interval showed a posivive correlation with changing value of colonic motility index (r, =0. 55,p<0.01). (4) Plasma concentration of motilin revealed a significant increase in the IBS group after MDT stress (p<0.01), and the value differed significantly from that of the control group (p<0.05). However, gastrin, glucagon, and substance P in the blood disclosed no significant change. 5) Stress interview caused significant elevation of colonic motility index in the IBS group (p<0.02) as well as MDT Ioading. (6) Administration of neostigmine and propranolol induced a significant increase of colonic motility index in the IBS group, which differed from that in the control group (p<0.01l, p<0.05). On the contrary, no significant difference was observed between IBS and control groups by administration of atropine, isoproterenol and naloxone. (7) According to the reaction under the experimental study, patients with IBS could be classified as follows ; (i ) An obviously stressrelated group whose functional disturbance is caused by psychological stress (psychosomatic disease). (ii) A no or little stressrelated group which has functional disturbance of the bowel constitutionally. (iii) A group which has abdominal complaints without functional disturbance (neurosis). These findings suggest that IBS is heterogenous and that the first group, which can be named as the psychosomatic disease group, revealed reasonable pathogenesis of IBS that hypermotility of intestinal smooth muscle is caused by abnormal excitation of the autonomic (mainly parasympathetic) nervous system, abnormality of some receptors, and/or hypersecretion of gastrointestinal hormones.