Hyponatremia commonly complicates the clinical course of patients with congestive heart failure (CHF) and seems to be an ominous prognostic factor. Angiotensin-converting enzyme (ACE) inhibitors improve the symptomatology of CHF patients. Moreover, it has been reported that these drugs can raise serum sodium in hyponatremic patients. The aim of this prospective work was to study the mechanisms involved in the correction of hyponatremia in 6 patients aged 52-69 years with CHF and hyponatremia (serum sodium 125– 128 mmol/l) who were receiving digitalis and furosemide. In these patients, captopril was introduced in progressively increasing doses. The drug induced significant clinical improvement. Additionally, a statistically significant increase in serum sodium was observed which was correlated to a rise in the diluting ability of the kidney (increase in C<sup>e</sup>H<sub>2</sub>O). A slight increase in creatinine clearance was also found, which could have contributed to the improvement in hyponatremia. Therefore, we conclude that ACE inhibitors can improve hyponatremia in CHF patients by increasing the urinary diluting ability.