Intestinal smooth myosin B was prepared from muscle layers around the lesion in four patienst with congenital intestinal atresia. And mypsin B ATPase activity was compared between the proximal dilated segment and the distal segment to atresia. In three, whose atresia might occur at an early period of gestation, proximal bowels showed less activity of myosin B ATPase than distal bowels. However, in another, whose atresia might occur due to intrauterine intussusception at a late stage of gestation, the proximal bowel indicate higher ATPase activity of myosin B that the distal bowel. These data meaned that contractibility of the proximal intestine to atresia was depressed in the former patients and was accelerated in the latter patient. These results suggested that the extensive resection of dilated proximal bowel in the congenital intestinal atresia is not always necessary to obtain good postoperative intestinal dynamics at the operation of the atresial lesions which may be induced at a late period of gestation.