Electrical stimulation of the abdominal vagi revealed that abdominal vagal fibres are distributed throughout the cervical vagal trunk. Conduction velocities of the main group of abdominal fibres ranged from 0.4 to 1.5 m.sec-1. 2. Thirty-seven single afferent fibres, with endings in the gastric and intestinal mucosa, were isolated from the cervical vagus of adult rats. Conduction velocities of eighteen of these endings ranged from 0.5 to 2.5 m.sec-1. 3. Light mechanical stimulation, such as stroking the mucosa, evoked a rapidly adapting response from these endings. Sustained pressure or distension with fluid or air did not excite these endings although excessive stretching of the mucosa excited three units. Balloon distension usually evoked an 'on-off' response due to phasic stimulation of the mucosa by the balloon during inflation and deflation. 4. These same endings also functioned as non-specific, slowly adapting chemoreceptors responding to various organic and inorganic acids, tap water and distilled water, alcohol, hypertonic saline, NaOH, NH4Cl, CuSO4, casein hydrolysate, mustard powder and cayenne pepper. Hypertonic glucose and guinea-pig bile were ineffective as stimuli. 5. In the presence of an excess of chloride ions the effectiveness of acids in stimulating these endings was in part determined by the pK of the acid. However pH per se was not the basic determining factor but rather the molecular size of the acid.
