Interest in prolonged perfusions has increased pari passu with the possibility of treating the more complicated cardiac deformities. Prolonged perfusions may be of interest also in functional disturbances arising during operations using the heart-lung machine, since it may be difficult to discontinue the perfusion before cardiac function is again satisfactory. Furthermore, the biological changes which take place during prolonged perfusion reflect, in an intensified degree, the changes that occur during an ordinary brief perfusion (Andersen, 1961). Even though the outcome is fatal after such a long-lasting perfusion, it is of interest to investigate the biochemical changes during the perfusion, especially if the patient has not succumbed to the perfusion. It is of even greater interest to ascertain whether the patient might have survived such a long-lasting perfusion had he not succumbed to other causes. A very prolonged perfusion in a patient who has survived has not yet been reported. This is not surprising, since prolonged extracorporeal circulation has usually been necessitated by a life-threatening condition. The data from a 12-hour perfusion are presented.