How animals manage their oxygen stores during diving and other breath-hold activities has been a topic of debate among physiologists for decades. Specifically, while the behaviour of free-ranging diving animals suggests that metabolism during submersion must be primarily aerobic in nature, no studies have been able to determine their rate of oxygen consumption during submersion (Vo₂D) and hence prove that this is the case. In the present study we combine two previously used techniques and develop a new model to estimate Vo₂D, accurately and plausibly in a free-ranging animal, using data from macaroni penguins (Eudyptes chrysolophus) as an example. For macaroni penguins at least, Vo₂D can be predicted by measuring heart rate during the dive cycle and the subsequent surface interval duration. Including maximum depth of the dive improves the accuracy of these predictions. This suggests that energetically demanding locomotion events within the dive combined with the differing buoyancy and locomotion costs associated with travelling to depth to influence its cost in terms of oxygen use. This will in turn effect the duration of the dive and the duration of the subsequent recovery period. In the present study, Vo₂D ranged from 4 to 28 ml min^-1 kg^-1, indicating that, at least as far as aerobic metabolism was concerned, macaroni penguins were often hypometabolic, with rates of oxygen consumption usually below that for this species resting in water (25.6 ml min^-1 kg^-1) and occasionally lower than that while resting in air (10.3 ml min^-1 kg^-1).