ABSTRACT. The role of electrolyte, carbohydrate, and base composition, a s well a s osmolality, of oral hydration solutions (OHS), was investigated using a nonabsorbable marker and tritiated water in an in vivo intestinal perfusion system in rats. The O H S tested were the World Health Organization recommended formula, containing 90 mEq/ liter sodium and 11 1 m M glucose, which was taken a s the reference solution; five variants of this solution with different sodium and glucose concentrations; and two solutions without sodium, i.e. isotonic glucose and deionized water. Also tested were one solution with acetate in lieu of bicarbonate, and two commercial preparations where citrate substituted for bicarbonate. The best water absorption rates were obtained with World Health Organization-type O H S characterized by a combination of low osmolality and moderate sodium and glucose content. Hypotonic O H S (190, 220, and 155 mosmol/kg) in which the sodium:glucose ratios were 60:30, 60:60, and 3055, respectively, produced mean jejunal water transport rates of 3.46, 3.20, and 2.91 pl/min/cm, respectively, whereas the standard World Health Organization O H S (330 mosmol/kg) resulted in a rate of 1.36 pl/min/cm ( p < 0.001). Similar good water absorption was achieved when Ac was the base (270 mosmol/kg and 60:111 sodium:glucose ratio) and with one of the commercial solutions (245 mosmol/kg and 50:111 sodium:glucose ratio). The reference World Health Organization O H S allowed for sodium absorption, a s did the O H S with sodium:glucose ratios of 90:45,60:30,60:60, and acetate-containing 60:111. Sodium a t a concentration of 30 mEq/liter or less resulted in the efflux of this electrolyte. High glucose concentration and lower osmolality exacerbated this effect. The results obtained in this investigation may assist in better evaluating O H S and in selecting modified formulae geared to specific hydration needs and possible replacement of water and sodium losses. (Pediatr Res 19: 894-898,1985 OHS, the cost factor, and other considerations (1-4). The most important points involved in the formulation of OHS have been sodium and carbohydrate concentrations and osmolality. The rationale for the amount of sodium, potassium, and chloride provided in the OHS has been related to the degree of potential electrolyte losses during the illness (5, 6).Several field and experimental studies have tested the suitability of the OHS proposed by the WHO, containing 90 mEq/liter sodium, or of a variant with a lower sodium content (7-10). It is generally accepted that both can accomplish rehydration, with the WHO formula also allowing for the replenishment of sodium. The amount of glucose added (20 g/liter or 1 1 1 mM) has been considered optimum.Earlier reports had provided conflicting information on the stoichiometry of sodium:glucose cotransport and the fluxes of electrolytes and water, either from in vitro (1 1-14) or from in vivo studies, including animal experimentation (15, 16) and clinical trials (I 6, 17).We previously evaluated the rela...