The assessment of parameters which adequately represent rectal and neorectal compliance is complex. Biological properties of the rectum during distension and relaxation show significant departures from in vitro physical compliance measurements; as much dependent upon the viscoelastic characteristics of hollow organ deformation as upon the technique of compliance calculation. This review discusses the pressure/volume characteristics of importance in the rectum during distension from a bioengineering perspective and outlines the disparities of such measurements in living biological systems. Techniques and pitfalls of newer methods to assess rectal wall stiffness (impedance planimetry and barostat measurement) are discussed. .