Heat loss leading to hypothermia is common during surgery, particularly when a body cavity is exposed. To determine the principal site of heat loss we used heat flux transducers to measure heat loss from the skin and the exposed abdominal cavity of seven pigs. Heat loss from the skin was 74 ± 15 W/m2, and from the abdominal cavity, 350 ± 122 W/m2(p< 0.002; ratio = 1:4.7). The area of the skin was 0.52 ± 0.06 m2, and of the abdominal cavity, 0.04 ± 0.006 m2(ratio = 13.6:1). For every 13.6 watts lost from the skin, 4.7 watts were lost from the abdominal cavity; i.e., skin heat loss accounted for 74% of total heat loss. Therefore adequate insulation would reduce the incidence of hypothermia. Evaporation accounted for the largest heat loss from the abdominal cavity. Evaporative losses could be minimised by enclosing the bowel in a plastic bag.