1. It is known that contraction of the upper esophageal sphincter (UES) and secondary peristalsis protect the airway and clear the esophagus of refluxed gastric contents. However, the exact nature of the stimulus and the role of acid remain controversial. 2. Secondary peristalsis and UES responses were measured following the intraluminal infusion of 0.1 N hydrochloric acid and equiosmolar NaCl solutions in seven normal volunteers. The protocol consisted of three phases: infusion of increasing volumes (1, 3, 5 and 7 ml per min), infusion of a given volume (7 ml per min) at different stimulation sites and balloon distension. 3. At the proximal esophagus the UES response to both solutions was similar, rising from a basal resting pressure of 30 mmHg to 70 mmHg for both HCl (range 60-85 mmHg) and NaCl (55-85 mmHg). The magnitude of the response decreased as the distance from the UES increased. The level of response decreased to 40 mmHg for both NaCl and HCl (range 30-60 mmHg) at the distal esophagus. These responses were also volume dependent. When the solutions were infused at 7 ml/min into either the proximal or distal esophagus, the sphincter pressure increased with increased volume to 65 mmHg for HCl (range 50-85 mmHg) and 60 mmHg for NaCl (range 50-80 mmHg). Secondary peristalsis was also induced by the two solutions. The level of response again decreased as the distance from the UES increased. This response was also volume dependent. Esophageal distension by a balloon positioned 10 cm below the sphincter induced secondary peristalsis and sphincter response identical to those induced by the infusion of fluid. 4. These results indicate that the principal stimulus for recruitment of the esophageal motor clearance mechanism is intraluminal distension and not necessarily the pressure of intraluminal acid.