Biomechanical properties of the rectal wall were studied in 17 healthy adult volunteers (nine men and eight women). With impedance planimetry it is possible to obtain simultaneous measurements of pressure and rectal cross sectional area (CSA) during balloon inflations. Rectal distensions were done with an intraluminal balloon using specified pressures up to 40 cmH2O above baseline rectal pressure. Balloon inflation elicited a phase of rapid increase in rectal This study concerns a method named impedance planimetry by which it is possible to obtain segmental information on biomechanical properties at one specific circumference of the rectal wall. The aim was to describe these properties by measuring rectal cross sectional area (CSA) at the site ofdistension upon graded isobaric distension in healthy volunteers. We have used the technique in human and animal studies of intestinal wall properties." Methods SUBJECTSWe studied 17 healthy volunteers without any history of bowel disease. The volunteers were nine men (age 28 (27-29) years, height 185 (179-187) cm, weight 80 (75-85) kg) and eight women (26 (23-29) years, 169 (166-172) cm, 58 (53-63) kg). Before the study we obtained informed consent from all subjects. The study was performed according to the second declaration of Helsinki and approved by the Ethical Committee of Aarhus county.The study was conducted between 8 00 am and 1 00 pm. There was a period of at least 12 hours of fasting before the investigations. No laxative was used and subjects were asked to empty the rectum and bladder. Before the investigation a digital examination confirmed an empty rectum. TECHNIQUE AND EXPERIMENTAL PROBE DESIGNWe used a specially constructed four electrode impedance measuring system located inside a balloon on a 20 cm long probe with an 8 mm external diameter (Fig 1). Two outer ring electrodes for excitation were placed with an interelectrode distance of 6 cm. A generator delivered a constant alternating current of 100 microamperes at 5 kHz for excitation.9 5 Midway between the excitation electrodes we placed two ring electrodes with an interelectrode distance of 0-2 cm for measurement of impedance. We used copper electrodes wound around the probe in 0 5 mm wide grooves to ensure a smooth surface. The distance between the excitation and detection electrodes was chosen on the basis that the CSA can be measured up to a diameter of 4 to 5 times the interelectrode distance. 3We measured CSA according to the field gradient principle from the measurement of impedance of the fluid inside the balloon.'3'I_8Rise time for measurement of CSA was 0-02