An electron study of histologically normal small intestinal mucosa taken from 10 children has shown, on morphological grounds, that the mid-region of the villus is best adapted for digestion and absorption. Microvilli in the mid-region were tallest, and presented the maximal surface area. In contrast the upper region of the villus exhibited a reduction in microvillous surface area and some cellular damage. Cellular extrusion was observed near the base of the villus as well as near the tip. The occurrence of cellular alterations in the oldest and more exposed regions of normal mucosa in childhood may be symptomatic of a natural epithelial ageing process but may also be the result of an adverse luminal effect on the enterocytes. These findings differ from some of the observations made on adult small intestinal mucosa where it has been reported that the microvilli present a maximal surface area and the enterocytes are most active at the tips of the villi rather than in the mid-region.