Adult hooded seals (Cystophora cristata) may dive for ~1hr and reach depths of 1000 m. To achieve this they store large amounts of oxygen bound to hemoglobin in the blood and to myoglobin in their skeletal muscles, and economize with these stores through cardiovascular and metabolic adjustments during dives. Their pups are born extremely precocial and display a rapid increase in diving duration and depth shortly after weaning, which takes place after a lactation period of only 3–4 days. Blood oxygen stores are at birth already at the level of adults, but muscle stores are only at 25% of the adult value. To investigate factors governing the development of myoglobin, muscle biopsies were collected at intervals from weaned captive hooded seal pups that were kept in an indoor pool where they could swim and dive (n=4), and on a snow ledge outdoors without access to water for swimming/diving (n=4), Samples were obtained from Musculus longissimus dorsi (LD), which is highly active during swimming but fairly idle when pups are out of water, and from the fore-flipper muscle M. supraspinatus (SSP), which is only sporadically used both in water and on land. Pool animals showed a rapid initial rise of myoglobin levels in LD, from 25.5±3.4 mg/g (n=4) to 43.7±6.4 mg/g (n=4) within the first month, and displayed significantly higher levels than in the non-diving group (F1,35 = 10.9, p =0.002). In contrast, myoglobin levels in SSP showed a slower increase and there were no differences between pool and outdoor animals (F1,34 = 1.034, p =0.316). These findings suggest that activity, rather than hypoxia, is the main trigger for the neonatal increase of myoglobin in seals. We also found that ~50% of the adult myoglobin level was reached already during their first month of life. This coincides with the rapid initial increase in dive duration of free living hooded seal pups, which suggests that the myoglobin level influences their diving behavior.