In acute experiments on anaesthetized rats from three age groups (15-20, 25-30 days of postnatal life, adult ones), studies have been made on water basins of the organism after peroral injection of hypertonic (2.5-5%) solutions of NaCl (5 and 10 ml per 100 g of the body weight). It was demonstrated that during ontogenesis, total content of water decreases mainly at the expense of extracellular fluid. Infusion of saline solutions into the stomach of rats decreases fluid content in all water basins, especially in the interstitial one. The level of changes depends on the volume of the injected solution and, to a greater extent, on the concentration of the latter and the age of animals. The described response is due to osmotic transport of water into the alimentary tract, as suggested by the decrease of water content in this tract. The role of the digestive tract in osmotic and volume regulation during peroral salt loading is discussed.