Immersion of the rat's hind limb in water at 45 degrees or 46 degrees C for one hour caused a severe burn of the extremity, including the popliteal lymph node. Pretreatment one day in advance at 45 degrees C for 15 minutes or 43 degrees C for one hour prevented most of the damage to lymph node and other tissues. The production of both thermal necrosis and thermotolerance were direct effects of heat on the lymph node. The distribution of necrosis in the lymph node (greatest in the subcapsular region and least in the medulla) probably relates to thermal gradients and not to the microstructure of the nodes, except perhaps for relative resistance of the medulla.