Hooded seals (Cystophora cristata) rely on large stores of oxygen, either bound to hemoglobin or myoglobin (Mb), to support prolonged diving activity. Pups are born with fully developed hemoglobin stores, but their Mb levels are only 25-30% of adults. We measured changes in muscle Mb concentration [Mb] from birth until one year of age in two groups of captive hooded seal pups, one being maintained in a sea-water pool and one on land during the first two months. All pups fasted during the first month, but were fed from there on. The [Mb] of the swimming muscle musculus longissimus dorsi (LD) doubled during the month of fasting in the pool group. These animals had significantly higher levels and a more rapid rise in m. longissimus dorsiLD [Mb] than those kept on land. The [Mb] of the shoulder muscle, m. supraspinatus, which is less active both in swimming and hauled-out animals, was consistently lower than in the m. longissimus dorsiLD and did not differ between groups. This suggests that a major part of the postnatal rise in LD [Mb] is triggered by (swimming) activity, and coincides with the previously reported rapid early development of diving capacity in wild hooded seal pups. Liver iron concentration, as determined from another 25 hooded seals of various age, was almost 10 times higher in young pups (1-34 days) than in yearling animals and adults, and liver iron content of pups dropped during the first month, implying that liver iron stores support the rapid initial rise in [Mb].