The economy of Cl-, K+, and Mg++, extracellular volume (ECV) and plasma volume, and the role of hyperreninemia and hyperaldosteronism were explored in 22 patients with congenital chloride diarrhea. Stool volume was in significant correlation with its Cl-, Na+and K+content, the correlation being significantly better with Cl-content than with the Na+content. Low fecal Cl-concentrations were seen in chronic hypochloremic contraction, but acute episodes did not cause reduction of fecal Cl-concentration from the basal level of 140–150 mmol/liter. The adequate condition (defined as normal serum electrolyte concentrations and blood pH, and presence of Cl-in urine) was associated with high total exchangeable Cl-and ECV. This excess Cl-and ECV roughly equalled the high daily fecal amount of Cl-and volume. Reduced ECV was accompanied by high renin activities and hyperaldosteronism, but in the adequate condition these were normal. Hyperaldosteronism caused a decrease in urinary Na+-K+ratio and, after the age of 2–6 months, in the fecal Na+-K+ratio. Total exchangeable K+was normal in the adequate condition. No Mg++depletion was present, although the patients lack Mg++substitution. The adequate condition could be maintained with an oral supplement of NaCl, KCl and water.SpeculationHyperreninemia, hyperaldosteronism, and hypokalemic alkalosis in congenital chloride diarrhea (CCD) are secondary to hypochloremic ECV contraction and can be completely corrected with adequate substitution. Mg++depletion does not play a part in CCD. The intestinal response to hyperaldosteronism appears later in the intestine than in the kidney in the newborn CCD patient and presumably also in the normal newborn.