The rectum can distend to accommodate stool, and contracts in response to distention during defecation. Rectal motor dysfunctions are implicated in the pathophysiology of functional defecation disorders and fecal incontinence. These rectal motor functions can be studied by intra-luminal measurements of pressure by manometry, or combined with volume during rectal balloon distention. Pressure-volume (p-v) relationships provide a global index of rectal mechanical properties. However, balloon distention alone does not measure luminal radius or wall thickness, which are necessary to compute wall tension and stress respectively. It has been suggested that the elastic modulus, which is the linear slope of the stress-strain relationship, is a more accurate measure of wall stiffness. Also, measurements of compliance may not reflect differences in rectal diameter between subjects prior to inflation, and imaging is necessary to determine if, as has been suggested, rectal pressure-volume relationships are affected by extra-rectal structures. We have developed a technique to measure rectal stress:strain relationships in humans, by simultaneous magnetic resonance imaging (MRI) during rectal balloon distention. After a conditioning distention, a rectal balloon was distended with water from 0 to 400 ml in 50 ml steps, and imaged at each step with MRI. The fluid filled balloon was segmented from each volume, the phase-ordered binary volumes were transformed into a geometric characterization of the inflated rectal surface. Taken together with measurements of balloon pressure and of rectal wall thickness, this model of the rectal surface was used to calculate regional values of curvature, tension, strain, and stress for the rectum. In summary, this technique has the unique ability to non-invasively measure the rectal stress:strain relationship and also determine if rectal expansion is limited by extra-rectal structures. This functional information allows the direct clinical analysis of rectal motor function and offers the potential for characterizing abnormal mechanical properties of the rectal wall in disease.