Weddell seals are well adapted polar animals. They give birth to pups in the Antarctic summer when they haul out onto the ice. While feeding their pups, they have a massive fasting weight loss (and fat mobilisation) and their suckling pup has a correspondingly massive weight gain and fat deposition. The amount of energy expeded by the mother seals during the suckling period was investigated. ... Different techniques were used to determine energy expenditure on two non-suckling adults and two suckling mother-pup pairs including weighing the seals, measuring body size (length and girth), measuring the thickness of fat/blubber under the skin using an ultrasound scanner, measuring the total body water as an index of fatness and measuring milk production over a 10-day interval. During the first season of study, this was carried out on two occasions with daily blood and milk sampling in between each animal. In the following season the measurements were repeated at intervals of about two weeks throughout the lactating period in 13 females. Twelve other seals and pups in the colony were just blood sampled at these intervals for measurement of biochemical markers of metabolism. Change in body composition (as a function of body water space), weight and size was determined in pups also to estimate how much of the maternal energy expenditure was retained by the pups.