Rectal sensation is usually assessed using balloon distention. The authors describe a more precise technique that avoids the variables of balloon dynamics and rectal diameter and compliance. Using both methods, rectal sensation was assessed in 13 healthy control women and 26 women with severe idiopathic constipation. Balloon distention in the rectum revealed an elevated sensory threshold (16.9 +/- 4.4 vs. 30.4 +/- 3.1 ml air, controls vs. patients, P = 0.018) and volume required to elicit a call to stool (61.1 +/- 9.1 vs. 97.5 +/- 6.4, P = 0.003) in subjects with severe constipation. The maximum tolerated volume was similar in the two groups. Rectal mucosal electrosensitivity was then tested using a bipolar ring electrode supplying a constant current. This demonstrated an elevated sensory threshold in the constipated subjects (16.3 +/- 3.0 vs. 27.4 +/- 2.1 mAmps, P = 0.005). There was a significant correlation between the sensation thresholds for balloon distention and electrical stimulation for the entire subject group (r = 0.39, P = 0.01). Two other patients with severe constipation after suspected extrinsic nerve damage were also studied and demonstrated a markedly abnormal electrical sensory threshold. Electrical testing avoided the variables inherent in balloon distention and was well tolerated, accurately quantifiable, and reproducible. The raised threshold to electrosensory mucosal testing suggests the presence of a rectal sensory neuropathy in patients with severe idiopathic constipation.