The incomplete information in the literature on the mechanism of sigmoid storage and evacuation and its relation to the rectum has prompted our study in 18 mongrel dogs. A condom-ended catheter was introduced into the sigmoid and infused with carbon dioxide (CO2) at 2 rates: slow and rapid. The pressure responses of sigmoid colon, rectosigmoid junction (RSJ), rectum and rectal neck to sigmoid distension were determined; the EMG activity of the external anal sphincter and levator ani was also evaluated. Upon slow sigmoid distension up to a mean of 93.6±3.2 ml, no pressure response occurred in the sigmoid, RSJ, rectum or rectal neck. Distension above this level induced an increase of sigmoid pressure (p 0.05); the balloon was dispelled to the rectum. EMG of external anal sphincter and levator showed no activity. Balloon expulsion to the rectum effected a pressure increase in the rectum (p 0.05), and the balloon was dispelled to the exterior. Rapid sigmoid distension induced an increase of sigmoid pressure (p<0.001) and a decrease of RSJ pressure (p<0.05) at approximately half the volume needed with slow distension. The sigmoid colon adapts the new contents until a certain volume is attained, when it contracts dispelling its contents to rectum. Rapid distension induces sigmoid contraction at a smaller volume than slow distension. Rectum and rectal neck did not respond to sigmoid distension or contraction, and vice versa. Sigmoid contraction initiates RSJ relaxation, while rectal contraction evokes RSJ contraction.