To evaluate effects of altering rectal contrast volume on defecatory effort during magnetic resonance defecography (MRD).We assessed defecation qualitatively and quantitatively as a function of rectal distention (group A: 180 cc, n=31; group B: 120 cc, n=31). Quantitative evaluation comprised measuring rectal area on midline sagittal images pre- and post-defecation.Resting rectal area was significantly higher for group A than for group B (35.2 vs. 28.3 cm2, P<.0001). Post-defecation rectal area and change in area (pre- to post-defecation) were not significantly different. Subjective evaluation showed no significant difference.Decreasing rectal gel volume from 180 to 120 cc did not compromise defecation performance during MRD.