The transport mechanism of D-xylose across the mucosal border of small intestine was investigated in humans both in vitro and in vivo. A comparative study was also made between infants and adults. We investigated the Na+ dependence of D-xylose influx from the mucosal solution into the epithelial cells, the phlorizin sensitivity of the influx, transport kinetics and electrogenic property of the D-xylose transport. The results of kinetic study indicated that the transport of D-xylose was completely diffusional. The influx of D-xylose across the mucosal border of human small intestine was Na+ independent and phlorizin insensitive. The electrical property of D-xylose transport could not be observed. There was no difference in the characteristics of D-xylose transport between small intestines of infants or adults. These results in the present study suggest that D-xylose does not share a common transport mechanism of D-glucose, and xylose tolerance test does not give any information about the absorptive capacity of D-glucose.