Southern elephant seals (Mirounga leonina) spend about ten month at sea, interrupted by two fasting periods on land. In October, females come ahore and give birth to a pup, suckle it, mate and return to sea. During the 23 days of lactation, the cows fast completely while the pups triple their birthweight solely on the milk diet. The weaned pups remain ashore and fast about 6 weeks while body tissues mature prior to start foraging at sea independently. From December to February, adult seals of both sexes return ahore for moulting, and fast again.Serial blood samples were collected from lactating females, weaned pups, and moulting adults of both sexes at the colony on King George Island during the two austral summers 1996 and 1997. To distinguish the biochemical aspects of fasting periods, serum concentrations of free fatty acids (FFA) and b-hydroxybutyrate (b -HBA) were measured photometrically. Both are key metabolites in the lipid metabolism.Curves of FFA- and b-HBA concentrations differed considerably. For lactating cows, weaned pups and moulting adults, a highly adapted permissible limit of fast is assumed. Cows showed an increase both of FFA- and b-HBA concentration. This is obviously due to the comparatively high energy expenditure of simultaneous lactation and fast. Weaned pups showed a marked increase in b-HBA but not in FAA. Moulting adults, however, did not differ significantly in the two parameters. The failured increase of FFA- and b-HBA concentrations may indicate that seals feed occasionally during the moult. Their fasting on land might be simply a consequence of a temporarily reduced ability of thermoregulation in water when skin perfusion and thus thermosteresis is increased. Exceeding the limit of fast may lead to pathological starvation and induce a strong feeding impulse. A cow may even leave its pup when such impulse dominates the maternal nursing instinct. Weaned pups then are forced to reduce the onshore maturation period which may cause insufficient foraging capability at sea. From our results we infer that thelevels of key metabolites such as FFA and b-HBA terminate the seal's natural fast on land in order to build up new fat reserves at sea.