The purpose of this study was to evaluate the mechanical factors involved in the genesis of lower esophageal sphincter pressure. We determined the relationship between intraluminal pressure and inside diameter, estimated the ratio between the wall thickness to inside radius, and calculated the tension in the wall of the lower esophageal sphincter as a function of the inside diameter. Various degrees of circumferential stretch were applied by introducing probes of different diameters in the rat lower esophageal sphincter in vivo. The intraluminal pressure produced by the lower esophageal sphincter around each probe was measured and pressure-diameter curves were constructed during (a) resting state, (b) contraction produced by electrical stimulation, and (c) relaxation produced by esophageal distension. The intraluminal pressure at an inside diameter of 0.5 mm was similar to that at inside diameter of 3.2 mm. This was true for the sphincter at rest as well as upon electrical stimulation. The pressure diameter curve, however, was sigmoid in shape; at first it showed a decline and then an increase followed by decline in pressure again with increasing diameters. The ratio of wall thickness to inside radius or the magnification factor varied with inside diameters as expected and this ratio increased steeply at small inside diameters. The tension diameter curves of the sphincter muscle showed that optimal tension development occurred not near sphincter closure but at a much wider diameter of 3.2 mm and that this muscle developed tension even at small luminal diameters. This behavior of the sphincter muscle ensures effective intraluminal pressure over a wide range of luminal diameters.