To investigate thermoregulatory adjustments at sea, body temperatures (the pectoral muscle and the brood patch) and diving behavior were monitored during a foraging trip of several days at sea in six breeding king penguins Aptenodytes patagonicus. During inactive phases at sea (water temperature: 4-7°C), all tissues measured were maintained at normothermic temperatures. The brood patch temperature was maintained at the same values as those measured when brooding on shore (38°C). This high temperature difference causes a significant loss of heat. We hypothesize that high energy expenditure associated with elevated peripheral temperature when resting at sea is the thermoregulatory cost that a post-absorptive penguin has to face for the restoration of its subcutaneous body fat. During diving, mean pectoral temperature was 37.6 ± 1.6°C. While being almost normothermic on average, the temperature of the pectoral muscle was still significantly lower than during inactivity in five out of the six birds, and underwent temperature drops of up to 5.5°C. Mean brood patch temperature was 29.6 ± 2.5°C during diving and temperature decreases of up to 21.6°C were recorded. Interestingly, we observed episodes of brood patch warming during the descent to depth suggesting that in some cases, king penguins may perform active thermolysis. It is hypothesized that functional pectoral temperature may be regulated through peripheral adjustments in blood perfusion. These two paradoxical features, i.e. lower temperature of deep tissues during activity, and normothermic peripheral tissues while inactive, may highlight the key to the energetics of this diving endotherm while foraging at sea.