The energy and material transferred from Northern elephant seal mothers to their pups during the 26.5-day lactation interval was measured using mass change and water influx data. The dilution of tritium- and deuterium-labelled water was monitored in six female elephant seals and their pups. During the nursing period, pup mass increased by 10.4% of the initial body mass per day while females lost 4.19% of their initial body mass per day. Over the entire nursing period, females lost 42.2±4.9% of their initial body mass. Maternal mass transfer to the pup was efficient; pups gained 54.9% of the mass lost by their mothers. Females lost only 14.9% of their lean mass compared to 57.9% of their adipose mass. The fasting metabolic rate of females was 66.2±7.5 MJ/day, 2.41 times the predicted standard metabolic rate (SMR) of a terrestrial mammal of equal size; the total energy output was 163±11 MJ/day, 5.98 times the predicted SMR. Energy consumption for the entire lactation interval was 4330±460 MJ. Milk production accounted for 60% of this energetic expenditure. This efficient milk production was made possible by temporal separation of lactation and feeding. Separation of feeding from lactation confers several energetic and ecological advantages: reduction in the metabolic overhead used for maintenance metabolism onshore, reduction of foraging effort required to meet the demands of lactation, since feeding can be spread out over a significantly longer interval, and a greater flexibility while foraging at sea due to the absence of the pup.