There is histological and functional evidence that the internal anal sphincter is abnormal in patients with idiopathic faecal incontinence. The in vitro responsiveness of the internal anal sphincter to noradrenaline (an important sympathetic neurotransmitter) and electrical field stimulation (known to stimulate the intrinsic innervation) has been studied. Muscle strips from eight patients with incontinence undergoing postanal repair and five controls undergoing resection for low rectal carcinoma were studied. The contraction-response curves for noradrenaline were significantly different, and the EC50, the concentration required to produce 50 per cent of maximum contraction, was higher in incontinent patients (P less than 0.001). Electrical field stimulation produced initial contractions in four of the control group which were blocked by phentolamine. This contraction was not present in the incontinent patients (P less than 0.01). These results indicate an abnormality in the adrenergic innervation of the internal anal sphincter in patients with idiopathic faecal incontinence.