Elephant seals are of interest for physiological and biochemical studies in relation to metabolic changes during the periods of prolonged fast during breeding and their ability to exist in the polar regions. Our interest is focussed on the conditions of serum lipoproteins, since elephant seals showe comparatively high levels of cholesterol but no evidence of atherosclerotic diseases. We used two methods to investigate the lipoproteins in lactating females, sucking pups, and weaned pups during breeding ashore.Field studies were carried out during austral summers 95/96 and 96/97 at King George Island, Antarctica. The body weight and blood samples were obtained repeatedly from 12 lactating (fasting) females and their sucking pups during the breeding period, and from 9 fasting pups during the weaning period. Adults were immobilised with ketamine. Very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL) were isolated from blood serum by ultracentrifugation (UC), and by fast performance liquid chromatography (FPLC). Cholesterol was measured both in serum, and in each fraction derived from UC and FPLC.The investigations are concerned with lipoproteins, but consider also the reliability of the methods used.The UC and the FPLC show a general agreement of cholesterol distribution throughout the lipoprotein spectrum. HDL-cholesterol is the predominant fraction. Compared with humans, the serrum cholesterol concentration is elevated. During lactation, cholesterol levels decreased in females, while concentartions in sucking pups increased significantly. In weaned pups, no significant changes of cholesterol were noted.Cholesterol is suggested to play an important role in seals. A high amount of disposable cholesterol might be necessary to ensure sufficient fluidity of cell membranes in blood vessels when elephant seals dive (over 1500m). It might also be necessary as a source of precursors for aldosterone synthesis in order to save water in lactating (fasting) females and in weaned (fasting) pups.