The neonatal period in the northern elephant seal (Mirounga angustirostris) differs markedly from that of most mammals. An intense 1-mo nursing period is followed by 2-3 mo during which seal pups consume neither food nor water. Because skeletal development in other species is rapid during this period, we determined the capability of elephant seals to continue skeletal growth under fasting conditions. We analyzed indirect markers of bone turnover [serum osteocalcin and alkaline phosphatase activity and urinary calcium and hydroxyproline (Hyp) excretion] in samples obtained sequentially from live pups throughout the fast. Serum osteocalcin was low at the beginning of the fast, peaked between 17 and 42 days, and then decreased to initial levels after 10 wk. Alkaline phosphatase activity increased linearly through 9 wk of fasting. Urinary calcium was relatively constant through 9 wk but decreased after 10 wk, whereas free and total Hyp decreased linearly. Clearance of 3H-labeled Hyp showed that virtually all (> 99%) free Hyp was oxidized and that only bound Hyp, for which there is no degradative mechanism, was excreted. Kinetics of 45Ca clearance, a direct measurement of mineral turnover, was virtually identical to that for young humans. Histomorphometry of ulnae obtained from pups which died of natural causes (usually trauma inflicted by adult males) indicated active surfaces (osteoid plus eroded) between 30 and 60% of total surface of highly porous bone containing < 60% bone area/tissue area. Thus all indicators of skeletal activity confirmed that young elephant seals continue skeletal development, notwithstanding the prolonged period during which they take in no food or water.