1. The effects of vagal efferent fibre stimulation on the smooth muscle of the lower oesophageal sphincter have been studied on the anaesthetized animal and on the isolated and perfused organ.2. In both muscle layers (longitudinal and circular) vagal stimulation elicits two types of electromyographic (e.m.g.) potentials: (a) excitatory junction potentials (e.j.p.s) where there is a depolarization of the smooth muscle fibres. E.j.p.s can give rise to spike potentials inducing a contraction of the sphincter; (b) inhibitory junction potentials (i.j.p.s) where there is hyperpolarization of the smooth muscle fibres, often followed by a transient depolarization which may initiate spikes (post-inhibitory rebound).3. Pure i.j.p.s are observed after atropine treatment which suppresses e.j.p.s. Under these conditions, a long lasting vagal stimulation induces a long duration hyperpolarization concomitant with an opening of the lower oesophageal sphincter followed after the cessation of stimulation by a powerful rebound leading to a strong contraction which closes the sphincter.4. Several arguments, pharmacological (action of acetylcholine (ACh), atropine and hexamethonium) and physiological (threshold and latency of responses) lead to the following conclusions. Preganglionic vagal fibres are cholinergic and they activate (a) intramural excitatory cholinergic neurones; (b) intramural non-adrenergic inhibitory neurones (purinergic neurones). Preganglionic fibres leading to inhibition have a higher threshold than those leading to excitation. Both excitatory and inhibitory pathways are interconnected inside the intramural network. In particular, activation of intramural inhibitory neurones, by relaxing the oesophagus orally to the lower oesophageal sphincter, inhibits intramural excitatory neurones and subsequently blocks vagal excitatory responses.5. Two functions may be attributed to the vagal extrinsic innervation: (a) closure of the lower oesophageal sphincter by maintaining the basal tone of the sphincter; this would imply that at rest the inhibitory control is supplanted by the excitatory one; (b) sphincter opening during swallowing by suppressing the excitatory stimulus and reinforcing the inhibitory one (it may be recalled that after bilateral vagotomy, swallowing is no longer followed by a relaxation of the sphincter).