The local heat tolerance of human skin is investigated by the use of a thermostat-controlled heat-probe of 1 cm2 contact area. 43 degrees C is the highest skin temperature which is tolerated for about 8 hours with no restricted blood flow. The safe time of heat exposure is decreased to less than one half on increasing the temperature by one degree. Heat application at 44 degrees C skin temperature produces strong thermal injury of the skin within 8 hours. The heat exposure time without causing thermal injury is shorter at restricted blood flow.