1. The kinin-forming activity of hind limb lymph and of plasma has been examined in rabbits before and after thermal injury.2. Neither plasma nor lymph contained much active kallikrein activity but the enzyme was evident in samples treated with glass or with acid.3. There was little or no increase in the activity of enzyme activated by glass after thermal injury, but an increase in the activity of enzyme activated by acid regularly occurred.4. There were two increases in the activity of enzyme activated by acid-one about 2 h and the other 4-6 h after thermal injury. They corresponded to increases in vascular permeability as indicated by increases in the concentration of lymph protein.5. There was considerably more kininogen in the lymph and plasma than was used in the assays of kallikrein activity, showing that the increased kinin-forming activity in lymph was not the result of the passage of kininogen from the plasma.6. The increase in activity in lymph was not usually accompanied by a similar increase in the plasma. However, an increase in the activity of enzyme activated by acid sometimes occurred in the plasma simply as a result of prolonged anaesthesia.7. It is suggested that whereas the enzyme activated by glass is a measure of prekallikrein, the acid activatable enzyme appears as a result of the dissociation of a kallikrein-inhibitor complex. An increase in the concentration of this complex is therefore an indication of the preceding activation of kallikrein.