The effect of diaphragmatic contraction and relaxation on the lower oesophageal sphincter (LOS) pressure was studied in 10 healthy volunteers. Pressures in the oesophagus, LOS, and stomach were measured in three phases. Phase I, end tidal expiration; phase II, subjects inspired to total lung capacity (TLC) and kept the airway open (sustained diaphragmatic contraction); and phase III, at TLC subjects relaxed against a closed glottis (diaphragm relaxed). The LOS pressure in phase II was significantly higher than in phase I, while in phase III it was significantly lower compared with phase I. There was a net increase in LOS pressure of 32 mmHg in phase II as compared to phase I. After phase II, as subjects moved into phase III, there was a sudden drop in the LOS pressure. The LOS pressure in phase II correlated with the transdiaphragmatic pressure in a linear fashion. These observations suggest that diaphragmatic contraction in man enhances the LOS pressure and this augmentation of sphincter tone may be a vital component of the antireflux mechanism.