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Na dependence of d-glucose and 3-O-methyl- d-glucose absorption was investigated using the isolated rabbit small intestine perfused through the lumen and the vascular bed, thus imitating in vivo conditions. No dependence of monosaccharide transport on luminal Na concentration was demonstrable if the lumen was perfused at low flow rate. Due to Na secretion, however, Na concentration in the lumen bulk phase, initially being zero, was raised to more than 20 mmol/l during the course of the experiments. Na dependence of sugar transport could be shown, however, if (1) Na secretion was decreased (by use of a vascular medium with low Na concentration) or if (2) unstirred layer thickness was reduced (by enhancement of luminal flow rate). Both conditions allowed the Na concentration near the brush border membrane to be controlled. The results provide an experimental explanation for the apparently low degree of Na dependence of monosaccharide absorption under in vivo conditions. [ABSTRACT FROM AUTHOR]