Allocation of energy to thermoregulation greatly contributes to the metabolic cost of endothermy, especially in variable ambient conditions. Weddell seal (Leptonychotes weddellii) pups are born in the austral spring in Antarctica, and must survive both on ice and in water, two very different thermal environments. This study examined energetic costs associated with development and thermoregulation in Weddell seal pups. We measured mass-specific metabolic rate in air (MR_A) for 8 pups every 2w, from 1–7 weeks old, and also mass-specific MR in water (MR_W), beginning at 3w. We determined when these two values were equivalent (MR_E), signifying when the two environments had the same metabolic cost. Additionally, we calculated thermal conductance, assessed molt status, and estimated body composition. MR_A declined from 9.98+/-2.5 ml O2 min^-1kg^-1 in 1-week-old pups to 6.65+/-1.31 ml O₂ min^-1 kg^-1 at 7-week-olds (mean+/-SD), with a high degree of individual variability. There was a significant effect of age on MR_A (F_3,27=3.484, p=0.029), such that MR_A was significantly lower at 7w than at 1w (p=0.043). MR_W declined from 3w (12.0+/-3.06 ml O₂ min^-1kg^-1) through 7w (8.18+/-2.29 ml O₂ min^-1kg^-1), but there were no significant differences among age classes. By 5w, MR_E was reduced for all but 2 pups, which suggests development of thermal capabilities between 3 and 5 weeks of age. While absolute MR in air followed an allometric relationship with mass and scaled with an exponent of 0.7191 (R²=0.5571), MR in water was not allometric with mass (R²=0.0002). Timing of MR_E did not vary with molt status or estimated body composition. Overall, our results indicate Weddell seal pups have developed their thermal capabilities in water by ~5 weeks of age, independent of the amount of lanugo fur or body composition.