Mass loss by females and changes in the body composition of their pups were measured during lactation in southern elephant seals, Mirounga leonina, at King George Island, South Shetland Islands. Mothers lost 10.78±2.07 kg/day (n=6), while theor pups gained 5.18±1.5 kg/day. The period measured (19±0.6 days) represented 86% of the whole lactation period. During the period measured females lost an average 31% of their weight after parturition. Of the total mass gained by pups (98.7±30.5 kg), 45% was water, 12% was protein, and 41% was fat. Fat reserves of pups, which represented less than 2% of their body weight at birth, increased to 31% at weaning. This constituted a mean gain of 45 kg of fat during the experimental period. At weaning, pups at King George Island showed greater fat reserves than those at other breeding sites, a fact which could contribute to their survival when they start to feed at sea.