OBJECTIVE Because the role of sympathetic innervation in the defecation mechanism is still vague and unidentified, this study was performed to investigate this issue. METHODS The effect of individual administration of alpha- and beta-adrenoceptor blocking agents (3 mg/kg phentolamine mesylate and 1 mg/kg propranolol hydrochloride, respectively) and the effect of thoracolumbar sympathectomy on anal and rectal pressures was studied in 13 mongrel dogs. Pressures were measured by a 2-channel microtip catheter. Bilateral thoracolumbar sympathectomy was performed by excising the sympathetic ganglia from T11 to L2. Incremental rectal filling using a rectal balloon with simultaneous anal and rectal pressure measurements was continued until balloon expulsion was achieved. RESULTS Rectal balloon distension with 30 to 40 mL of saline affected rectal pressure increase (P < 0.001 ), anal pressure decrease (P < 0.01), and balloon expulsion. Following administration of either phentolamine or propranolol or after thoracolumbar sympathectomy, rectal pressure declined (P < 0.05), but anal pressure showed no change (P > 0.05). At a rectal balloon volume of 50 to 60 mL of saline, rectal pressure increased (P < 0.001), anal pressure decreased (P < 0.01), and the balloon was expelled. CONCLUSIONS Sympathetic rectal innervation may have a role during both the filling and evacuation phases of the defecation mechanism. During rectal filling, it most likely maintains rectal compliance. During evacuation in cases of rectal sympathetic block or denervation, a larger volume than usual of rectal distension is needed to induce rectal contraction and evacuation.