The anatomy and physiology of the external anal sphincter, puborectalis muscle, internal anal sphincter, and rectum are described. Measurement techniques are reviewed emphasizing those which can be carried out by the gastroenterologist in independent office practice, and the range of normal values for each test is given. A protocol for diagnostic evaluation of anorectal disorders is recommended which includes testing for rectal prolapse and for descent of the perineum by having the patient strain while seated on a commode chair, evaluating the external and internal anal sphincters by recording EMG with an anal plug, or recording pressures with a perfused catheter or balloon probe, using a rectal balloon to evaluate the maximum tolerable volume of rectal distension and the minimum (threshold) volume of a bolus injection which can be subjectively perceived, and arranging with a radiologist to perform a proctogram--a radiograph of the rectum and anal canal during rest and when the patient strains to defecate--in order to evaluate the anorectal angle (puborectalis muscle) and to detect rectocele and intussusception. These objective tests should be supplemented by a careful clinical history and physical examination.