Heart rate () and oxygen consumption () were recorded from gentoo penguins walking on a variable-speed treadmill. After an initial increase, and increased linearly with walking speed. A curvilinear regression equation best described the relationship between and . Over a 3-d period, and were recorded continuously from six gentoo penguins that had been injected with doubly labeled water (DLW). When was used to predict over the 3 d, the estimate uwas not significantly different from the measured with an error (-1.0%) that was less than that produced by the DLW method (+ 1.6%). The estimate produced by the technique was also accurate when applied both to shorter periods (30 min) and to periods of activity and inactivity. Seven gentoo penguins were also swum in a water channel, and and were again recorded. The relationship between and determined from the birds in the water channel was not significantly different from that obtained from the walking birds. It is seems reasonable that, since the relationship of most behaviors of the penguin have been determined while the animal is walking and swimming, can be used to predict the metabolic rate of free-ranging animals.