The mode of absorption of L-glucose and D-xylose was studied in vitro in preparations of hamster small intestine. Both sugars were actively transported from the lumen into the tissue when present at low concentrations. Their transport was Na+-dependent and was inhibited by ouabain, phlorizin, and competing sugars. Vmax for both sugars was Na+-independent and identical with that for many other monosaccharides. The apparent Km was only 5 times (D-xylose) or 8 times (L-glucose) less in the presence than in the absence of Na+(in contrast to 130 times for D-galactose). This explains why active transport is limited in extent and observable only at low concentrations. Countertransport was demonstrated with both these weakly transported sugars in the presence of strongly transported sugars, e.g. D-glucose, which explains the apparent secretion of L-glucose in the kidney in vivo. It is concluded that the mode of transport of L-glucose and D-xylose is identical with that of other sugars and consistent with a model proposed earlier.