The effect on anal tone of electrical stimulation of the presacral (hypogastric) sympathetic nerves has been studied in eight patients during abdominal rectopexy or restorative proctocolectomy. A sharp fall in anal pressure occurred in seven patients (mean fall 59 cmH2O; range 35-80 cmH2O). In one patient given a beta- and alpha-sympathetic blocking drug (labetalol 200 mg) intra-operatively, the anal pressure decreased by 15 cmH2O. These observations show that stimulation of the presacral sympathetic nerves causes relaxation of the internal anal sphincter and implies that these nerves may induce relaxation of the sphincter in vivo. The pathway of the recto-anal reflex has been studied intra-operatively in three patients undergoing rectal excision. The recto-anal reflex is present after presacral nerve blockade and after full mobilization of the rectum, but is abolished by circumferential rectal myotomy. The reflex has a local intramural pathway. This observation validates the assumption that absence of this reflex is a feature of aganglionosis, as in Hirschsprung's disease.