The pathophysiology of idiopathic constipation and of the irritable bowel syndrome is complex and not easily approached through clinical data alone, none of which appearing to be highly specific of these diagnoses. Abnormal visceral sensation as well as colonic and ano-rectal motor dysfunction have been demonstrated. Functional evaluation may then focus on establishing the presence of such abnormalities or on trying to understand their underlying mechanisms. Thus, for example, measuring colonic transit time may distinguish those patients with constipation and a prolonged transit time from those with a normal one. Ano-rectal manometry may be used to evaluate the recto-anal inhibitory reflex which is absent in Hirschsprung's disease, although its use in patients with idiopathic constipation requires further study. Electromyography (EMG) of the anal sphincter can be used to demonstrate a paradoxical contraction during defecation, which is thought to be an important mechanism in some types of constipation. Manometry an EMG are useful tools in evaluating the patient with severe constipation, particularly if surgery is contemplated.