&NA;To obtain clear evidence of the rectoanal reflex, cold water was injected into the rectum. All patients with a normal reflex had a rectoanal reflex when the water was either 4°C or 17°C. The same reflex was seen when a balloon was used for distention. No reflex was evident when the water was 37°C. At 27°C or 45°C, the reflex was evident occasionally. In patients with Hirschsprung's disease, the rectoanal reflex was absent with balloon distention, and when water at any temperature was injected. As cold water will induced the rectoanal reflex safely and without rectal distention, measurement of the reflex using cold water is useful when the presence or absence of the reflex is doubtful. The authors' results suggest that the receptor related to the reflex is neuronal in origin and not muscular, and that the receptor locates near the mucosa.