Grey seal pups (Halichoerus grypus) were collected at the time of weaning (early November) and starved for 31 days at thermoneutrality. During starvation body weight decreased linearly, whereas metabolic rate was stable at 1.58 +/- 0.13 (SD) W X kg-1. Metabolic rate as related to body weight was significantly higher (P less than 0.01) than predicted by Kleiber (The Fire of Life, New York: Kreiger, 1975). Respiratory quotient averaged 0.67 +/- 0.03 during the 1st wk but increased to an average of 0.76 +/- 0.04 during the final week of fasting. Body composition was evaluated three times during starvation by use of computed tomography. The area of blubber and skeletal muscle in three transverse (thoracic, abdominal, pelvic) sections of the animals decreased on average 27.6 +/- 4.8 and 18.6 +/- 8.5%, respectively, from days 3 to 31 of fasting. Caloric content of blubber and skeletal muscle was determined by bomb calorimetry, and the caloric content of catabolized tissue was estimated. Based on this information it was calculated that approximately 94% of the energy expended by grey seal pups during the first 4 wk of the postweaning fast is derived from their subcutaneous deposits of fat.