Pinnipeds rely primarily on oxygen stores in blood and muscles to support aerobic diving; therefore rapid development of body oxygen stores (TBO(2)) is crucial for pups to transition from nursing to independent foraging. Here, we investigate TBO(2) development in 45 harp (Pagophilus groenlandicus) and 46 hooded (Cystophora cristata) seals ranging in age from neonates to adult females. We found that hooded seal adults have the largest TBO(2) stores yet reported (89.5 ml kg(-1)), while harp seal adults have values more similar to other phocids (71.6 ml kg(-1)). In adults, large TBO(2) stores resulted from large blood volume (harp169, hood 194 ml kg(-1)) and high muscle Mb content (harp 86.0, hood 94.8 mg g(-1)). In contrast, pups of both species had significantly lower mass-specific TBO(2 )stores than adults, and stores declined rather than increased during the nursing period. This decline was due to a reduction in mass-specific blood volume and the absence of an increase in the low Mb levels (harp 21.0, hood 31.5 mg g(-1)). Comparisons with other phocid species suggests that the pattern of blood and muscle development in the pre- and post-natal periods varies with terrestrial period, and that muscle maturation rates may influence the length of the postweaning fast. However, final maturation of TBO(2) stores does not take place until after foraging begins.