In congestive heart failure (CHF), pulmonary gas exchange, as evaluated by carbon monoxide diffusion (DLCO), is impaired. ACE-inhibition improves DLCO. Infusion of saline worsens DLCO, because of upregulated sodium and water transport to the alveolar interstitium, which thickens the alveolar-capillary interface and lengthens the diffusion path for gas exchange. We investigated whether enalapril can readjust the capillary permeability to sodium