It was observed previously (Cs aky , T.Z. and Fischer, E. (1981) Diabetes 30, 568-574), that sustained hyperglycemia enhances the intestinal transport of aldohexoses ; on the other hand, hyperfructosemia affects primarily the transport of fructose. The present study examines in detail the hyperketosemia -induced intestinal ketose transport. Intravenously infused 3-O- methylfructose produces marked 3-O- methylfructosemia without concomitant hyperglycemia; in such animals the intestinal transport of both fructose and 3-O- methylfructose increased. The hyperketosemia -induced increased ketose transport was inhibited by phloretin but only if placed on the serosal compartment. Phlorizin affects neither the basal nor the induced intestinal ketohexose transport. The enhancement of the intestinal ketohexose transport is not sodium-dependent and is not inhibited by ouabain.