Concentration and metabolic replacement (turnover) rate of glucose and ketone bodies were determined at intervals during a 52 day postweaning fast in five grey seal (Halichoerus grypus) pups, using bolus injections of radiotracers. Blood glucose was maintained at a high level throughout the fast, while beta-hydroxybutyrate increased 26 times from day 3 to day 37, whereafter it by and large was maintained. Glucose replacement rate decreased to 56% of the day 9 value at day 37 and dropped further to only 32% of the day 9 level at day 52 in two seals, while in another 2 seals it increased at this late stage. The average ketone body replacement rate ranged between 8.6 and 13.8 mumols min-1 kg-1, but did not change significantly (P greater than 0.05) during the fasting period. These results suggest a reduced gluconeogenesis from protein and increased production of ketone bodies, which may in part replace glucose as energy source during fasting.