The relationship between the length of the external anal sphincter (EAS) and the anal pressure was observed in 37 guinea pigs. The results showed that if the muscle was stretched from 100% to 300% of its original length, the resting anal pressure was kept relatively steady (steady phase), but the actively contracting pressure was proportionally increased; if from 300% to 370%, a sharp increase of resting anal pressure was developed (accelerating phase), but the actively contracting pressure would gradually decrease to zero; beyond 370%, the resting anal pressure remained unchanged (plateau phase), while in the muscle, the nerve fiber, muscle fiber and blood vessel were damaged and necrotic. However, if the muscle was continously stretched at the accelerating phase of the resting anal pressure for 6 days, the muscle could grow in length at a rate of 2mm/d. In the growth muscle, the muscle cell was hypertrophic, the Schwann's cell in the nerve fiber was generated and the blood vessel became abundant. Both transversely split sarcomere and longitudinally split sarcomere were seen in the muscle cell in electron microscope. This result has surgical importance for treatment of anorectal anomaly.