1. The food consumption of an animal. both at the individual and the population level, is an essential component for assessing the impact of that animal on its ecosystem. As such, measurements of the energy requirements of marine top-predators are extremely valuable as they can be used to estimate these food requirements. 2. The present study used heart rate to estimate the rate of energy expenditure of gentoo penguins during the breeding season. The average daily metabolic rate (ADMR) of penguins when one adult was necessarily present at the nest (incubating eggs or guarding small chicks; IG, 4.76 W kg(-1)) was significantly lower than that when both parents forage concurrently during the major period of chick growth (CR: 6.88 W kg(-1)). 3. The ADMR of a bird was found to be dependent on a number of factors, including the day within the breeding season and the percentage time that the bird spent foraging during that day. 4. When they were ashore, the estimated metabolic rate of IG birds (3.94 W kg(-1)) was significantly lower than that of CR birds (5.93 W kg(-1)). However, the estimated metabolic rates when the birds were at sea during these periods were essentially the same (8.58 W kg(-1)). 5. The heart rate recorded when the penguins were submerged (128 beats min(-1)) was significantly higher than that recorded from resting animals when ashore (89 beats min(-1)). However, it was lower than that recorded from birds that were swimming in a water channel (177 beats min(-1)). This might indicate that, although primarily aerobic in nature, there was an anaerobic component to metabolism during diving. An alternative interpretation is that the metabolic requirement during diving was lower than when the birds,were swimming with access to air. 6. There was a significant decline in abdominal temperature. from 38.8 degreesC at the start of a diving bout to 36.2 degreesC at the end, which may, indicate a reduction in overall metabolic rate during submersion. This in turn may explain the lowered heart rate. 7. In the present study. we have shown that the metabolic rate of the gentoo penguin varies during the breeding season. The relatively constant metabolic rate of the birds when at sea could represent an upper physiological limit that the birds are unable to exceed. If so, it will only be possible for the birds to increase foraging effort by diving more frequently and/or for longer periods thus reducing their foraging efficiency (the energy gained during foraging vs. energy spent gaining that food). During years when food is scarce, this reduction in foraging efficiency may have a profound influence on the reproductive productivity of the gentoo penguin.