The absorption of glucose (free, and released from membrane hydrolysis of maltose) and water in the isolated loop of the rat small intestine was studied in chronic experiments. Even at maximum glucose (75 mmol/l) or maltose (37.5 mmol/l) concentrations the rate of glucose transfer by solvent drag and by diffusion did not exceed 13% and 25%, respectively, of the total rate of glucose absorption. Electron microscopic and immunocytochemical analysis revealed a significant widening of intercellular gaps in the basal epithelium region and an increase of actin density in the vicinity of the tight junctions and between the apical root filaments in enterocytes after glucose or glycine load in chronic and acute experiments. However, very rarely (in 1% of all cases), and only in chronic experiments, structural changes in the tight junctions such as "blisters" and dilatations were also recognised. It is concluded that under normal physiological conditions the absorption of glucose (free, and released from maltose hydrolysis) mainly uses active transport across the apical membrane of the enterocytes.