The presence of a sphincter at the rectosigmoid junction (RSJ) is debated. This investigation studies the presence or absence of a sphincter and its possible role in sigmoid colon storage and rectal evacuation. Eighteen healthy volunteers (10 males, 8 females) with a mean age of 36.6 2 14.8 years (range 21-53) were studied. The pressure response of the sigmoid colon, RSJ, and rectum to sigmoid and rectal distension, respectively, was determined before and after anesthetizing either the sigmoid colon or the rectum. The RSJ length was evaluated by the station pull-through technique. Sigmoid distension with balloon volumes of up to 80.6 2 4.4 ml of H 2 0 effected no sigmoid, RSJ, or rectal pressure changes ( P > 0.05). At a mean sigmoid distension of 88.6 2 4.1 ml of H20, the sigmoid colon showed a significant pressure increase ( P < 0.001), a RSJ pressure decrease ( P < 0.05), and insignificant pressure changes in the rectum ( P > 0.05); the balloon was dispelled into the rectum. Rectal distension of 94.6 ? 5.8 ml of H 2 0 produced rectal ( P < 0.001) and RSJ ( P < 0.05) pressure increases. Distension of the anesthetized sigmoid and rectum did not produce pressure changes in the RSJ (P > 0.05). This study demonstrated a high pressure zone at the RSJ of 3.8 ? 0.7 cm in length. This suggests that the RSJ might act as a functional sphincter. It opens reflexly upon sigmoid contraction, by a reflex we call "rectosigmoid inhibitory reflex," and closes upon rectal contraction, a reflex we call "rectosigmoid excitatory reflex." The former allows the stored feces in the sigmoid colon to pass to the rectum, and the latter reflex prevents stool reflux to the sigmoid upon rectal Contraction.