Resting anal pressure as determined by manometry is unsuited for assessing the competence of a sphincter. To validate a compliance method of sphincter evaluation we investigated the response of the anal canal to distension. In 20 healthy subjects the anal sphincter was distended using a cylindrical balloon probe subjected to continuous inflation. Two speeds of distension were used: 12 and 80 ml/min. Deflation, at the rate of inflation, immediately followed the inflation at 12 ml/min, and at 80 ml/min was performed after a deformation volume of 12 ml was maintained for 2 min. The response of the anal sphincter to 12 ml/min distension was characterized by an initial phase of resistance followed by relaxation of the anal sphincter in all subjects. In contrast, at 80 ml/min no opening was obtained. During the deformation a biomechanical relaxation curve was recorded. We conclude that the anal sphincter acts as a low-pass filter and not only as a pressure barrier. The response of the anal sphincter to distension is a simple test, which provides functional information on the behavior of the anal sphincteric zone.