The role of electrolyte, carbohydrate, and base composition, as well as 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 OHS tested were the World Health Organization recommended formula, containing 90 mEq/liter sodium and 111 mM glucose, which was taken as 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 OHS characterized by a combination of low osmolality and moderate sodium and glucose content. Hypotonic OHS (190, 220, and 155 mosmol/kg) in which the sodium:glucose ratios were 60:30, 60:60, and 30:55, respectively, produced mean jejunal water transport rates of 3.46, 3.20, and 2.91 microliter/min/cm, respectively, whereas the standard World Health Organization OHS (330 mosmol/kg) resulted in a rate of 1.36 microliter/min/cm (p less than 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 OHS allowed for sodium absorption, as did the OHS with sodium:glucose ratios of 90:45, 60:30, 60:60, and acetate-containing 60:111. Sodium at a concentration of 30 mEq/liter or less resulted in the efflux of this electrolyte. High glucose concentration and lower osmolality exacerbated this effect.(ABSTRACT TRUNCATED AT 250 WORDS)