The weights of harp seal pups quadruple during 13 days of suckling while hooded seal pups double in weight in a lactation period of just 4 days. Pups of both species then fast for a month or longer. As a first measure of tissue responses to this 'feast and famine' pattern, we weighted the body, sculp (blubber and attached skin), core (carcass including viscera) and major internal organs of seal pups at birth, at the end of suckling, and at the end of the fast. When expressed as a percentage of body weight, the weights of the internal organs of newborn harp and hooded seals were within the range reported for newborn land mammals. During suckling, harp and hooded seals gained 2.3 and 6.5 kg/day body weight, respectively, but a large part (64-73%) of this gain was blubber and skin rather than core. Even though pups were ingesting great quantities of fat, their digestive organs (stomach, small and large intestines, pancreas) were neither particularly large at birth nor did these organs gain in weight or length unusually rapidly. Most organs increased in weight in proportion to the increase in core weight, but the liver and spleen increased proportionately more than the core, and the stomach, heart and kidneys increased proportionately less. At the end of suckling, sculp accounted for more than half of the body weight in both species. The subsequent 4-week fast resulted in weight loss from both the sculp and core, and the liver and spleen decreased in weight by about 70%. The net effect of sequential suckling and fasting was particularly striking in the hooded seal pup, which has a lighter core, heart, liver and spleen at 1 month postpartum than at birth. These data illustrate a remarkable cycle of nutrient deposition and depletion which is undoubtedly central to the survival of young seals in the harsh pack-ice environment.