Although both energy metabolism and ventilation parameters are highly size dependent in adult birds, the usual scaling relationships with mass do not hold in hatchlings or in growing chicks. In Adélie penguins, three distinct behavioral phases during growth and development are reflected in the ontogeny of metabolism and ventilation. Metabolic intensity increases in the nestling phase (age 0-11 days), stabilizes during the creche phase (14-40 days), and decreases in fledglings (40-55 days) and adults. Minimal respiratory frequency does not change with mass in nestlings, decreases in creche chicks, and falls abruptly in fledglings and adults. Mass-specific standard tidal volume and mass-specific standard minute volume do not change with mass in nestlings, increase abruptly at 14 days, decline with increasing mass in creche chicks, and are not correlated with mass in fledglings and adults. Oxygen extraction at minimal frequency increases with increasing mass in nestlings and in creche chicks, and it declines with increasing mass in fledglings and adults. At any given age, usually Adélie penguins accommodate changing thermogenic demand primarily by adjusting minute volume rather than oxygen extraction.