We studied the effects of increased intra-abdominal pressure on the lower esophageal sphincter (LES) pressure in 15 healthy subjects. The role of the diaphragm in the genesis of LES pressure during increased intra-abdominal pressure was determined by measuring diaphragm electromyogram (EMG). The latter was recorded using bipolar intraesophageal platinum electrodes that were placed on the nonpressure sensing surface of the sleeve device. We also measured the LES pressure response to increased intra-abdominal pressure during inhibition of the smooth muscles of the LES by intravenous atropine (12 micrograms/kg). Straight-leg raising and abdominal compression were used to increase intra-abdominal pressure. Our results show that the increase in LES pressure during straight-leg raising is greater than the increase in gastric pressure. During abdominal compression, the rate of LES pressure increase is faster than that of the gastric pressure, suggesting an active contraction at the esophagogastric junction. The increase in LES pressure during periods of increased intra-abdominal pressure is associated with a tonic contraction of the crural diaphragm as demonstrated by EMG recording. Atropine inhibited the resting LES pressure by 50-70% in each subject but had no effect either on the peak LES pressure attained during increased intra-abdominal pressure or tonic crural diaphragm EMG. We conclude that 1) there is an active contraction at the esophagogastric junction during periods of increased intra-abdominal pressure and 2) tonic contraction of the crural diaphragm is a mechanism for this LES pressure response.