We evaluated the ability of low doses of botulinum A toxin, an inhibitor of acetylcholine release at the neuromuscular junction, to denervate and relax the spastic rhabdosphincter in 11 men with spinal cord injury and detrusor-sphincter dyssynergia. Toxin concentration, injection volume, percutaneous versus cystoscopic injection of the sphincter and number of injections were evaluated in 3 treatment protocols. All 10 patients evaluated by electromyography after injection showed signs of sphincter denervation. Bulbosphincteric reflexes in the 10 patients evaluated after injection were more difficult to obtain, and they showed a decreased amplitude and normal latency. The urethral pressure profile in the 7 patients in whom it was measured before and after treatment decreased an average of 27 cm. water after toxin injections. Post-void residual urine volume decreased by an average of 146 cc after the toxin injections in 8 patients. In the 8 patients for whom it could be determined toxin effects lasted an average of 50 days. The toxin also decreased autonomic dysreflexia in 5 patients.