Objective: To study the main distrib ution and the influence of S2？S4 nerves on urethral sphincter, detrusor u rinae and penis. To supply the theoretical guidance for the treatment of spas tic bladder after spinal cord injury with differential sacral rhizotomy. Methods: Twelve dogs were respectively given electrical stimulation to S2？S4 nerve roots. The contraction of urethral sphincter and detrusor uri nae, the change of bladder pressure and penis erection were examined and recorded. The peak value of evoked motor unit potentials of the corresponding muscles w ere compared. Results: Electrical stimulation to S2 nerve root induc ed peak value of evoked motor unit potential of detrusor urinae and change of bladder pressure which amounted to 60%of that under normal condition. Electrica l stimulation to S3 nerve root induced peak value of evoked motor unit potential of musculus sphincter urethrae which amounted to 57%of that under no rmal condition. Electrical stimulation to S4 nerve root induced improved penis er ection. Conclusion: S2？S4 nerves are the main innervating nerves to detrusor urinae, urethral sphincter and penis respectively. This finding helps to further rehablitate and recover the function of the lower urinary tract a fter spinal cord injury and avoid unnessesary functional damage in the course of treatment.