The influence of D-glucose on electrolyte and water transport in the distal ileum and proximal colon of the rabbit in vitro and in vivo has been studied. The magnitude of electrolyte and water transport was found to differ in vitro and in vivo. The replacement of mannitol by D-glucose in the intestinal lumen had no effect on ileal and colon transport of the ions studied, i.e., movement of the ions was independent of the supply of exogenous glucose. The distal ileum and the proximal colon of the rabbit in vivo showed a net secretion of bicarbonate, aimed at neutralizing the cecal contents.