The anterior compartment of the leg of 4 volunteers was used to study the transmission of externally applied pressure into a limb. The pressure from a pneumatic splint was found to be transmitted directly into the leg and was found to be added to the resting intramuscular pressure. The median inflation pressure of such a splint applied by trained ambulance personnel was 25 mm Hg (range 15-36 mm Hg). When the resting intramuscular pressure was raised experimentally, either by voluntary muscular contraction or venous engorgement, numerical addition of externally applied pressure was again observed. A moderate rise in compartment pressure commonly occurs after injury. Due care should be taken in the use of pneumatic splints or compressive dressings as the total pressure generated within a limb may be sufficient to induce ischaemia.