The metabolic rate of moulting macaroni penguins Eudyptes chrysolophus was investigated using three techniques in order to test two hypotheses concerning the energetic cost of the moult fast in penguins. First, that energy expenditure during the moult is greater than that while penguins are onshore during the breeding season. Second, techniques that do not measure energy expenditure throughout the moult fast do not accurately determine the true cost of the moult. Mass loss calculations, measurement of the rate of oxygen consumption and estimation of the rate of oxygen consumption from heart rate in the field were used with captive and free-ranging penguins. Comparison of the results from these techniques suggest that metabolic rate is higher in the field than in a respirometer due to an increase in thermoregulatory costs. Furthermore the average metabolic rate of female penguins during the moult at 5.04±0.85 W kg−1 was not significantly different from that of female penguins on-shore during the breeding season at 6.27±0.38 W kg−1. Metabolic rate in the field changed during the moult fast as feather loss and increased vascularisation of the skin caused increased heat production, illustrating the importance of determining energy expenditure from animals in the field throughout the whole of the moult fast.