Dogs were bled to a mean arterial pressure of 35 mm. Hg and maintained at this level for 4 to 5 hours, or until 40 per cent of their maximum bleeding volume was taken back spontaneously. When the superior mesenteric artery was autoperfused at normal rates with arterial blood, only 2 of 22 animals survived. On the other hand, when a donor animal was included in the perfusion system, a significantly higher (P=0.03) fraction survived (5 out of 14). If the removed blood was not reinfused as soon as 40 per cent of the maximum bleeding volume was taken back but the hypertensive period was maintained for a full 5 hours, then only 1 dog out of 9 survived as long as 35 hours, despite the fact that all 9 dogs in this group were given additional transfusions equal to the “take-up” volume.