1. The effects of streptozotocin-induced diabetes mellitus on active jejunal glucose uptake in vivo, and on galactose movement across the brush-border (phlorhizin-sensitive) and basolateral (phlorhizin-insensitive) membranes of isolated upper and mid-villus enterocytes has been studied. 2. Chronic diabetes increased unidirectional phlorhizin-sensitive galactose uptake by mid-villus but not upper villus cells. In contrast, phlorhizin-insensitive uptake by both cell populations was enhanced by diabetes. 3. Diabetes increased glucose absorption in vivo by mechanisms which were unrelated to hyperphagia. Mucosal hyperplasia acting together with an epithelium containing a higher proportion of mature enterocytes is the most likely explanation for the response. 4. We conclude that, during diabetes, the mid-villus region is an important site of adaptation with functional changes occurring at both the brush-border and basolateral membranes. The increased hexose transport ability of the basolateral membrane is retained during cell transit along the villus.