In order to investigate the vagal afferent pathway responsible for the previously reported effects of cholecystokinin (CCK) on gastric emptying and food intake, single afferent fibres were recorded from the cervical vagus of urethane-anaesthetized ferrets. Sixty tension receptor afferents with receptive fields in the corpus, antrum, duodenum, jejunum and ileum all showed a resting level of discharge which was augmented powerfully by distension of the segment containing the ending. Close intraarterial injection of CCK-8 (100-200 pmol) caused relaxation in proximal regions, but enhanced contractile activity in more distal regions. Mechanoreceptor discharge closely followed intraluminal pressure at all times, indicating a sensitivity primarily to tension and no direct sensitivity to CCK. Only duodenal tension receptors were significantly excited by CCK (due to increased contractile activity), whereas those in the stomach showed a net decrease. Thirty-seven mucosal receptors from the corpus, antrum, duodenum and jejunum showed responses to luminal stimuli: predominantly light stroking, acidity and hypertonicity as has been previously described. No responses to glucose or amino-acid infusions could be evoked. However, mucosal fibres showed a strong sensitivity to close-intraarterially injected CCK-8 (3-200 pmol) in 19/26 fibres tested. These responses were unaffected by cholinergic blockade when tested. The data strongly suggest that in the ferret only vagal mucosal receptors are directly sensitive to CCK-8. These fibres are therefore likely candidates for mediating some of the reflex and behavioural effects of CCK when it is released from the gastrointestinal tract and acts directly on vagal sensory endings.