Small bowel adaptation after extensive intestinal resection has been studied in 5 children. A segmental perfusion technique was used to evaluate the kinetics of glucose absorption and sucrose hydrolysis. The activity of 6 brush border hydrolases and the mean villus height were measured in intestinal biopsies. Glucose absorption was greater than in 3 control children of the same age and was almost complete. Sucrose hydrolysis was increased in the same proportion and the maximum capacity of the segment was 2 mmol/min/20 cm as compared to 1.2 mmol/min/20 cm in the control group. Absorption of the released monosaccharides was complete for glucose and as high as in controls for fructose. Brush border hydrolase activities were identical to those of controls whereas the mean villus height was increased (438 micrometer as compared to 342 micrometer) although the difference was not significant. These results demonstrate the functional adaptation of the remaining small bowel and confirm that it is due to compensatory hyperplasia.