SUMMARY1. A modification of the vascular perfusion technique was used to investigate sugar transfer from the lumen of the rat small intestine to the vascular bed.2. With this method the transfer of both D-[3H]galactose and L-[14C]glucose were followed in the same experiments. The results indicate that when equimolar concentrations of the two sugars are perfused through the lumen there is a preferential transfer of galactose from the mucosal epithelium to the vascular bed as well as from lumen to the epithelial layer.3. At high rates of galactose transfer from the lumen to the vascular bed, accumulation of the sugar within the mucosal epithelium is minimal unless the vascular flow is stopped.4. Phlorizin inhibits galactose transfer from the lumen to the vascular bed, but the inhibition of sugar transfer appears to be restricted to the luminal face of the epithelial cells since the selectivity of the basolateral exit process is unaffected.5. Replacement of Na+ by K+ in the luminal perfusate reduces the rate of galactose transfer by a factor of 40, but does not completely abolish the preferential transfer of galactose. The presence of luminal Na+ also stimulates the exit of galactose from the mucosal epithelium to the vascular bed.6. The presence of D-glucose in the vascular perfusate produces a two-to threefold increase in the rate of sugar transfer from the lumen to the vascular bed. This effect is not observed with vascular galactose or luminal glucose.

1. Sugar absorption by foetal and neonatal rat intestine has been examined using an in vitro accumulation technique. 2. The capacity for active sugar absorption is present in the rat intestine by the 17th day of gestation. Considerable variation in uptake occurs during the first month post partum; the greatest uptake place during the first week. 3. The absorption of sugars by the developing gut resembles in several respects that by the mature intestine. However, the adult pattern of functional localization along the intestine is not fully established until after the 3rd week post partum. 4. Kinetic studies indicate that variations with age in the distribution of sugar ‘carriers’ along the intestine, rather than changes in the ‘carriers’ themselves, account for the observed variations in absorption.

SUMMARY1. A modification of the everted sac technique is described which allows several sacs to be prepared rapidly and simultaneously from the same segment of rat intestine.2. A method has been developed for comparing the transport of two sugars by measuring changes in the ratios of their concentrations as they pass across the intestinal wall.3. With this method significant differences were observed between the D-[3H]galactose and L-["C]glucose ratios in the mucosal epithelium, the serosal tissue and the serosal compartment. These results indicate that both the efflux of galactose from the serosal side of the mucosal epithelium and the uptake of the sugar into the mucosa are carriermediated processes.4. The mediated efflux of galactose at the serosal side of the epithelial layer is inhibited by the presence of phlorizin on the mucosal side and to some extent by any reduction in the mucosal Na+ concentration. Both of these treatments inhibited galactose uptake at the brush border. Serosal efflux of the sugar appeared to be saturated at high concentrations of D-galactose.5. Pre-treatment of the sacs with mercuric chloride considerably reduced D-galactose uptake from the luminal side, but did not affect its efflux relative to L-glucose at the serosal side of the mucosal epithelium.6. Carrier-mediated sugar uptake into the mucosal epithelium from the serosal side was also examined. The role of the bidirectional, carriermediated sugar transport processes at the serosal pole of the mucosal epithelial cell in transintestinal transport is discussed.