Recently it has been morphologically clarified that a layer of thick smooth muscle, mimicking the internal anal sphincter, exists at the rectal and even in high and intermediate anorectal malformations. This paper will describe two studies focussed on the internal anal sphincter in anorectal malformation. I. Experimental study: Utilisation of the rectal end for reconstructive surgery would necessitate the mobilisation of the rectum. We performed a chronic animal experiment in order to investigate the influence of rectal mobilisation on anal function and the role of the internal anal sphincter in anal continence. Fifteen dogs were operated on; rectal separation was performed in 5 dogs, resection of the internal anal sphincter in 5 dogs and both procedures in 5 dogs. Anorectal manometric studies for 24 weeks revealed that rectal separation caused only a transient mild disturbance in anal functions with the exception of long-standing high rectal compliance, while resection of the internal anal sphincter caused a persistent severe disturbance. II. Clinical study: The function of the smooth muscle thickening at the rectal end was investigated and operations to preserve the rectal end were evaluated in cases of high and intermediate anorectal malformations. In 5 infants with anorectal malformations (high type 2, intermediate type 3), for whom colostomies had been performed as newborn, a preoperative manometric study at the rectal end was performed with a probe introduced from the distal colostomy. Thereafter, they all underwent a rectal end preserving operation. They were followed up manometrically and clinically after the operation. A preoperative manometric study of the rectal end showed the presence of rhythmic activity in all and positive reflexive pressure fall by rectal distension in 4. The rectal end preserving operation maintained rhythmic activity in all of them, rectoanal reflex in 3 and good results are expected clinically. These two studies suggest that the functional internal anal sphincter, which exists at the rectal end also in high and intermediate anorectal malformations, should be utilised for reconstruction because of the essential role it plays in anal continence.