The question was examined as to whether or not lower esophageal sphincter pressure (LESP) rises in response to increases in intragastric pressure. Pressure profiles of the lower esophageal sphincter (LES) were recorded with low compliance rapid pull-through manometry, in 9 healthy volunteers without hiatal hernia. Fundic pressure was increased by inflating the stomach with air. Air insufflation was stopped when gastric distension became painful (at 8.4 mm Hg +/- 0.7 SEM). No subject had nausea. Mean resting LESP was 24.6 mm Hg +/- 2.2 SEM. There was a negative linear relationship between fundic pressure and LESP: LESP decreased by 1.10 +/- 0.15 SEM per 1 mm Hg fundic pressure rise. On the average, the sum of fundic pressure and LESP remained constant. Thus, there is not only a lack of reflex contraction of LES in response to fundic pressure rise, but actually a weakening of the LES by fundic distension. This mechanism might facilitate belching following swallowing of air.