Twenty-two patients were studied before, early after (2 to 4 weeks), and late after (6 to 12 months) open intracardiac surgery. Their urinary sodium excretion was measured during an eight-day period of oral sodium loading and the results compared with normal values previously established by Braunwald and colleagues. Eight patients had normal urinary sodium excretion preoperatively. Two of six studied late postoperatively had abnormally low urinary sodium excretion at that time, both having had abnormally low excretion when studied early postoperatively. Fourteen patients had abnormally low urinary sodium excretion preoperatively. Nine of the patients from this group had normal sodium excretion when studied late postoperatively. Early postoperatively, 3 of 13 patients studied had abnormally low sodium excretion: one died three months later; one still had abnormally low excretion when studied late postoperatively; and one was not studied late after operation. No correlation existed between urinary sodium excretion and cardiac output, creatinine clearance, or intracardiac pressures. Abnormally low sodium-excreting ability early postoperatively is a poor prognostic sign. Most patients with impaired sodium-excreting ability preoperatively regain normal ability late after intracardiac repair of their mechanical defect.