Skeletal muscle necrosis is quantified using technetium-99m stannous pyrophosphate (99mTc-PYP) in pressurized muscle compartments after severe blood loss. Six dogs (15 to 20 kg) were anesthetized by pentobarbital sodium (25 mg/kg IV) and hemorrhaged to a hypotensive state. Left hind-leg muscle compartments were pressurized to a level of 20 mm Hg for 6 hours by infusing autologous plasma. Intracompartmental pressure was continuously monitored by the wick catheter. The right leg served as a control. Forty-eight hours following pressurization, 5 mCi of99mTc-PYP were injected IV, and 3 hours later each dog was sacrificed and pressurized and control muscles were resected simultaneously, weighed, and counted for99mTc-PYP uptake. Significant uptake appeared in muscle compartments pressurized for 6 hours at 20 mm Hg, indicating that 20 mm Hg in a hypotensive state produces a degree of necrosis as great as that produced by 40 to 50 mm Hg in a normotensive state. We conclude that an acute compartment syndrome occurs in a hypotensive individual at an intramuscular pressure level considerably less than the threshold pressure level in an individual with normal blood pressure.