AIM: To investigate whether there was a dominant sacral root for the motive function of rectum and anal sphincter,and to provide an experimental basis for sacral root electrically stimulated defecation in spinal cord injuries.METHODS: Eleven spinal cord injured mongrel dogs were included in the study. After L4-L7 laminectomy, the bilateral L7-S3 roots were electrostimulated separately and rectal and sphincter pressure were recorded synchronously. Four animals were implanted electrodes on bilateral S2 roots.RESULTS: For rectal motorial innervation, S2 was the most dominant (mean 15.2 kPa, 37.7% of total pressure),S1 (11.3kPa, 27.6%) and S3 (10.9 kPa, 26.7%) contributed to a smaller part. For external anal sphincter, S3 (mean 17.2 kPa, 33.7%) was the most dominant, S2 (16.2 kPa,31.6%) and S1 (14.3 kPa, 27.9%) contributed to a lesser but still a significant part. Above 85% L7 roots provided some functional contribution to rectum and anal sphincter.For both rectum and sphincter, the dght sacral roots provided more contribution than the left roots. Postoperatively, the 4 dogs had electrically stimulated defecation and micturition under the control of the neuroprosthetic device.CONCLUSION: S2 root is the most dominant contributor to rectal pressure in dogs. Stimulation of bilateral S2 with implanted electrodes contributes to good micturition and defecation in dogs.