The dynamics of the rectal detrusor at defecation was studied in 29 normal and 10 constipated subjects. The rectal pressure was measured by a ureteral catheter with terminal side holes, while the intra-abdominal pressure was measured by a Foley catheter introduced into the urinary bladder. Simultaneous recording of fecal flow was performed by fecoflowmetry. Assessment of the findings in normal subjects has shown a pre-evacuation rectal pressure elevation which reached its peak with the start of fecal flow and then fell gradually as the flow increased. This was in contrast to the constipated patients in whom the elevated pre-evacuation rectal pressure continued through the whole act of evacuation. Quantitative analysis of the rectal pressure curve could also differentiate between normal and constipated subjects. The significant parameters are the opening pressure and time, the pressure time and the evacuation pressure. The simultaneous recording of the intra-abdominal pressure demonstrates its share in the increase of the rectal pressure.