Both vagal and sympathetic innervation been have described as influencing hormone release from the gastrointestinal tract and pancreas. The role of neural influences on the release of gastrin, glucagon, and secretin has been studied using the potent autonomic nerve stimulus of hypoglycaemia. Healthy subjects were each rendered hypoglycaemic by insulin 0.2 units/kg on three occasions: after atropine 20 microgram/kg: after propranolol 160 mg orally, and without prior drug administration. Adequate beta-blockade was confirmed by observation of the pusle rate response to a standard exercise at the end of the experiment, and by measurements of plasma propranolol levels. Hypoglycaemia failed to produce a rise in plasma gastrin under either propranolol or control conditions but a significant rise was noted with prior atropinisation. The glucagon response to hypoglycaemia, when measured with either the C- or N-terminal reactive antibodies, was found not to be influenced to any significant extent by either beta-blockade or atropinisation. No alteration in plasma secretin levels was noted during hypoglycaemia. It therefore appears that neural influences are relatively unimportant in the release of gastrin, glucagon, and secretin in man.