Radioisotopes allow accurate quantitation of the pattern and effectiveness of the transit of chyme through the small and large intestines. Abnormalities of small bowel transit can be demonstrated in patients with the irritable bowel syndrome, and patients with chronic idiopathic intestinal pseudo-obstruction due to either a visceral myopathy or neuropathy. In the colon, radioisotopic studies of transit have demonstrated the site of delayed transit in some severely constipated patients. In patients with these disorders of transit, functional studies may influence the choice of medical or surgical therapy although there are few prospective studies which have established their worth in this context. Radioisotope studies can also be utilised to study the effectiveness of delivery of drugs to the small and large bowel, and to study the adequacy of rectal evacuation in patients with a defaecatory disturbance. The low radiation dose and possibility of frequent observations make radioisotope studies valuable for clinical and research studies in functional gastrointestinal disorders.