To optimise the composition of oral rehydration solutions (ORS) for European children is not a simple task. Although controlled clinical trial is ultimately the only way to determine whether a new solution is superior to an established ORS, testing many different formulations is neither feasible nor ethical. Several groups of investigators have evolved the concept of using animal models to test new ORS formulations. Disease-related animal models using perfusion of cholera toxin-treated rat small intestine or experimental rotavirus infection of neonatal rats suggest that optimal water absorption will be obtained by using a hypotonic ORS with a sodium concentration of 50-60 mmol/l and a glucose concentration of 50-100 mmol/l. Addition of citrate or bicarbonate had no benefit with respect to the promotion of water absorption.