The inhibitory nerve fibres in the vagal supply to the guinea-pig stomach.

1. A comparison was made between the inhibitory responses of the isolated atropinized guinea-pig stomach to stimulation of the vagus and to stimulation of the perivascular nerves.2. When low frequencies of stimulation were used, the vagi were more effective than the perivascular nerves in causing relaxation of the stomach.3. The responses to vagal stimulation were faster in onset than were the responses to perivascular nerve stimulation.4. Responses to perivascular nerve stimulation were abolished by low concentrations of bretylium, whereas responses to vagal stimulation were only slight reduced. Vagal responses were strongly reduced by higher concentrations of bretylium.5. Amphetamine relieved the blockade of perivascular nerve responses which was caused by bretylium, but never relieved the reduction of the vagal responses.6. It is argued that the vagal inhibitory fibres are not susceptible to ;adrenergic neurone blockade', the observed reduction of the responses being due to some other action of bretylium.7. These differences between the vagal and perivascular inhibitory innervations of the stomach suggest that the post-ganglionic fibres in the vagal pathway and the intramural inhibitory nerves described in the guinea-pig caecum are similar.