Weaning mass in southern elephant seals is highly variable, the heaviest pups being three times as heavy as the lightest ones. After weaning, pups undergo anextensive postweaning period in which they draw on their reserves. To quantify the energy expenditure during the postweaning period, changes in mass, bodycomposition, and postweaning duration were measured in southern elephant seals at King George Island, South Shetland Islands, Antarctica. Overall, mean pupweaning mass was 154 ± 26 kg (n = 117) and did not differ between sexes. Mean minimum postweaning duration was 42.5 ± 7.5 d. Heavier animals at weaning hadlower mass-specific mass loss rates than lighter ones, and a faster depletion of body reserves was associated with a shorter postweaning period. The proportion ofbody mass represented by fat at weaning was 37% ± 4% (n = 47) and did not differ between sexes. Of these pups, 36 were recaptured after a mean period of 36 dafter weaning. On average, total mass loss measured in these animals (39 kg) was composed of 39% water, 47% fat, and 12% protein. The composition of massloss was not significantly different between sexes and was not related to weaning mass or total body energy reserves. However, fatter animals at weaning lost morefat per kilogram lost than thinner ones. Late in the fast, males and females appeared to be in a similar body condition. Nevertheless, the overall proportion of bodymass represented by fat at this time was lower than that presented by the same animals at weaning. We estimated that during the postweaning period pups lost, onaverage, 30% of their mass at weaning. This comprised approximately 35% of the energy and 32% of the fat in the pup's body.