Hypotonic oral rehydration salts solutions (ORS) have been proved to be better than isotonic solutions with respect to water absorption. To establish whether a base precursor is essential in the composition of a hypotonic ORS with improved absorption properties, a randomised double blind clinical trial was conducted comparing two formulas of hypotonic ORS, each with an osmolality of 224 mmol/l, with or without citrate, in a group of 107 children admitted to hospital with acute diarrhoea. The two solutions were effective in the correction of dehydration and there was no difference between the treatments in the duration of diarrhoea. The patients receiving the hypotonic ORS with citrate consumed less of the solution, however, and their metabolic acidosis was corrected earlier. It is concluded that citrate is clinically advantageous in a hypotonic ORS, but a hypotonic formula without a base precursor is also effective.