PURPOSE: This study was undertaken to investigate the effects of anal stretching on anal pressures and damage to the external anal sphincter. METHODS: This study was performed on 37 guinea pigs. Animals were divided into three groups: control group, quick stretching group, and continuous overstretching group. Anal stretching was conducted by an 8-F Foley(R) catheter balloon. RESULTS: It was found that if the muscle was stretched from 100 to 300 percent of its original length, anal resting pressure (ARP) kept relatively steady, but anal contracting pressure (ACP) gradually increased; from 300 to 370 percent, a sharp ARP increase was developed, but ACP gradually decreased to zero; beyond 370 percent, ARP remained unchanged (plateau phase). By histologic examination, it was revealed that when the muscle was stretched at the ARP plateau phase, an ischemic zone of necrosis and an edematous zone of necrosis could be clearly identified in the muscle. CONCLUSION: This study shows that length of the external anal sphincter definitively influences muscle strengths, and severe anal stretching will result in muscle damage. These results imply that the sphincteric muscle complex in high or intermediate anorectal anomalies may be injured during present conventional surgical approaches.