An oral sodium tolerance test was devised to detect and quantify impaired Na excretion in patients with heart failure. Following a 4-day period during which daily Na intake was limited to 10 mEq., 80 mEq. of Na were administered daily for 4 days and 150 mEq. for another 4 days. The total urinary Na excretion during the 8-day test period was determined. Thirteen normal subjects excreted between 550 and 734 mEq. of Na. Of 41 patientswith heart disease, 31 excreted subnormal amounts of Na, between 10 and 550 mEq. and only 10 excreted normal quantities. An alternate plan, in which 320 mEq. of Na was administered in 8 days, was also utilized in patients in whom the standard 920-mEq. Na load was considered inadvisable. There was no correlation between the impairment of Na excretion, as estimated by the Na tolerance test, and the etiology of the heart disease, the glomerular filtration rate, or any hemodynamic variable. Patients with marked functional disability excreted less Na than those with few symptoms, and the patients with the most severe impairment of Na excretion all had a history of edema. Striking improvement in Na tolerance followed corrective cardiac operations. The Na tolerance test was found of particular usefulness in the evaluation of patients in whom clinical examination and hemodynamic studies gave no conclusive evidence as to the presence or absence of congestive heart failure.