The water and energy metabolism of 12 free-living gray seal pups during their postweaning fast was investigated using tritiated water for determination of rates of energy expenditure and water turnover and changes in body composition. Changes in serum composition and urinary output were also measured. The duration of the postweaning fast ranged from 10 to greater than 28 days. The mean daily energy expenditure of pups was 12.0 +/- 1.0 (SE) MJ, equivalent to 2.7 times the basal metabolic rate predicted from M. Kleiber (The Fire of Life. Huntington, NY: Krieger, 1975). On average, lipid catabolism provided 94% of the energy used. Water-turnover rates were lower than predicted from C. R. Richmond, W. H. Langham, and T. T. Trujillo (J. Cell. Comp. Physiol. 59: 45-55, 1962). Urinary output of water and solute were also low, and these declined as the fast progressed. Although pups were in negative water balance, there was no evidence that they experience dehydration during the post-weaning fast. The principal adaptations to prolonged fasting in weaned gray seals are, therefore, lowering of field metabolic and water-turnover rate and reliance on stored lipid as fuel.