During the Antarctic winter emperor penguins fast for up to 120 days when breeding at rookeries, which may be as much as 120 km from open water. Emperors have lost almost half of their body mass by the time they walk back to the sea to feed. Resting metabolic rate and metabolic rate during treadmill walking at 1.4 km times h-1 were measured regularly along the course of 63-118 days of fasting in four emperors that lost between 33 and 55% of their body mass. Resting metabolic rate decreased linearly with body mass throughout the fast; it was 76 and 50 W at 39 and 18 kg body mass, respectively, which therefore corresponds to a limited increase in the resting metabolic rate per unit of body mass. There was a considerable decrease in the metabolic rate for walking at 1.4 km times h-1, from 340 to 140 W at body masses of 39 and 18 kg, respectively; this decrease was linear with body mass but at a steeper rate below 23 kg. From 39 to 23 kg, the cost of walking per unit of body mass remained constant. Below 23 kg (a point where about 2.5 kg of fat remain), the increased efficiency for walking may be due to a change in the mechanics of locomotion.