Studies of arterial blood gases and acid-base equilibrium in twenty infants in congestive heart failure have revealed the following alterations: (1) elevation of arterial Pco2, (2) diminution of arterial Po2, (3) lowered pH. These findings demonstrate the presence of respiratory insufficiency in infants with pulmonary congestion and cardiac decompensation. Studies of serum electrolytes have shown in some infants a hyponatremia and hypochloremia present prior to the use of therapeutic agents and these alterations may influence response to digitalis and diuretic agents.