To compare the diving capacities of juvenile and adult emperor penguins Aptenodytes forsteri, and to determine the physiological variables underlying the diving ability of juveniles, we monitored diving activity in juvenile penguins fitted with satellite-linked time/depth recorders and examined developmental changes in body mass (Mb), hemoglobin concentration, myoglobin (Mb) content and muscle citrate synthase and lactate dehydrogenase activities. Diving depth, diving duration and time-at-depth histograms were obtained from two fledged juveniles during the first 2.5 months after their depature from the Cape Washingon colony in the Ross Sea, Antarctica. During this period, values of all three diving variables increased progressively. After 8-10 weeks at sea, 24-41 % of transmitted maximum diving depths were between 80 and 200 m. Although most dives lasted less than 2 min during the 2 month period, 8-25 % of transmitted dives in the last 2 weeks lasted 2-4 min. These values are lower than those previously recorded in adults during foraging trips. Of the physiological variables examined during chick and juvenile development, only Mb and Mb content did not approach adult values. In both near-fledge chicks and juveniles, Mb was 50-60 % of adult values and Mb content was 24-31 % of adult values. This suggests that the increase in diving capacity of juveniles at sea will be most dependent on changes in these factors.