Background:Congenital chloride-losing diarrhoea is characterized by a defect in chloride/bicarbonate exchange, which is normally present in the ileum and colon. Whether the defect is an absence or a reversal of such an exhange is unclear, and we have investigated two young children with the disorder to answer this question.Methods:We used a previously described nonequilibrium rectal dialysis method, using different dialysate anion concentrations, to investigate the movement of sodium, potassium, chloride, and bicarbonate in the rectum of the two children.Results:The results showed that chloride and bicarbonate movements were not linked in any active way, and both ions appeared to move passively in response to the electrochemical gradients generated.Conclusions:In the two subjects studied, the defect in the rectum appears to be an absence of the normal anion exchange mechanism present in the bowel, rather than its reversal.