The influence of feeding isocaloric diets containing either 65% of fructose (F 65) on 65% of glucose (G 65) were studied on the uptake of both sugars in segments of rat proximal jejunum and distal ileum. The hexose absorption was compared to that obtained in animals receiving isocaloric amounts of a diet containing 30% of glucose (G 30). Feeding fructose (F 65) for 3 days resulted in a 2.5-fold increase of fructose uptake in the jejunum and a 40% increase in the ileum as compared to group G 30. When fructose (F 65) was administered instead of G 65 the uptake of fructose was enhanced by 75% in the jejunum and 35% in the ileum. Stimulation of glucose absorption in segments of the proximal and distal small intestine by diets F 65 and G 65 was nearly identical as compared to the values of group G 30. The stimulation of the uptake of fructose induced by fructose feeding parallels an adaptive increase in the activity of enzymes involved in fructose metabolism in the mucosa of the small intestine.