This study tests the hypothesis that reperfusion injury is the principal cause of limb loss after acute arterial occlusion and that this injury is avoidable. Of 61 isolated hindlimbs amputated at the level of the hip joint, 17 were controls (group I), 5 were perfused without ischemia to establish the validity of the model (group II), and 15 underwent 4 hours of ischemia at room temperature without reperfusion (group III). Acute embolectomy was simulated in 24 limbs after 4 hours of ischemia; 12 were reperfused with standard Krebs-Henseleit solution at 100 mm Hg (group IV), and 12 were reperfused under controlled conditions (i.e., 37 degrees C, 50 mm Hg) with substrate-enriched modified reperfusate (group V). Leg volume, water content, contractile function, and high-energy phosphate content were assessed and data were expressed as mean +/- SD. Four hours of ischemia caused a profound fall in adenosine triphosphate content (4.0 vs 26.0 mmol/L/gm of protein, p less than or equal to 0.001). Uncontrolled reperfusion resulted in severe reperfusion injury; massive edema developed (83% vs 75%, p less than or equal to 0.01), leg volume increased markedly (21.5% above control, p less than or equal to 0.001), and no contractile function followed electrical stimulation. In contrast, controlled reperfusion resulted in normal water content (76.9% vs 75.0%, NS) and minimal change of leg volume (5.5% +/- 5% of control, NS), replenished adenosine triphosphate completely (24.2 vs 26.4 mmol/L/gm of protein, NS), and restored immediate contractile function in all limbs (24.3% +/- 14% of control). This study shows that 4 hours of room-temperature ischemia (18 degrees C) does not produce irreversible damage of the rat hindlimb because the reperfusion injury that follows uncontrolled reperfusion can be avoided. Immediate recovery of contractile function can be restored if the conditions of reperfusion are controlled by gentle reperfusion pressure (50 mm Hg) at 37 degrees C and if a modified substrate-enriched, hyperosmotic, alkalotic, low-Ca++ reperfusate is administered.