The dog jejunum is a much denser tissue than the ileum, with a greater weight per unit length and higher proportion of mucosal tissue. Morphometric analysis reveals longer and wider villi, deeper crypts and larger enterocytes in the jejunal mucosa. Uptake of phenylalanine or β-methyl-glucoside by tissue slices in vitro is slightly greater in jejunal than in ileal tissue. The levels of acid phosphatase and alkaline phosphatase in the individual enterocytes are significantly greater in the jejunum, according to quantitative histochemical analysis. The absorption of water, sodium, potassium, chloride and glucose in vivo is significantly smaller in the jejunal than in ileal loops, particularly when expressed in terms of unit mucosal weight. Sodium and water absorptions are stimulated by glucose at both sites, but the stimulation is significantly greater in the ileum. Opposite results have been obtained in rats where the transport of phenylalanine in vitro is greater in the ileum, but water, electrolyte and glucose absorption in vivo is greater in the jejunum