Oxygen consumption ($\dot{V}_{O_{2}}$) and body temperature ($T_{b}$) were examined in 2-20-day-old Peromyscus leucopus exposed as individuals to ambient temperatures ($T_{a}$) of 0-35 C for 2.5-3.0 h. The same measures were taken on 4-14-day-old litters of four young enclosed in a cotton nest during exposure for 2.5-3.0 h to $T_{a}s$ (measured outside the nest) of 0-30 C. Among 2- and 4-day-olds tested as individuals (without a nest) some animals at 30 C $T_{a}$ maintained $T_{b}$ 3.7-4.7 C above $T_{a}$, and their $\dot{V}_{O_{2}}$, was 2.9-3.7 times higher than the $\dot{V}_{O_{2}}$, of animals at 35 C $T_{a}$. In 2- and 4-day-olds tested individually $T_{b}$ fell with $T_{a}$ as $T_{a}$ was lowered from 30 to 0 C anal closely approximated $T_{a}$ in tests at 0-15 C; $\dot{V}_{O_{2}}$, fell with $T_{a}$ below 25 C $T{a}$. Individual 18- and 20-day-olds, in contrast to 2- and 4-day-olds, usually maintained $T_{b}$ above 36 C when tested at 0-35 C $T_{a}$, and their $\dot{V}_{O_{2}}$, increased steadily ...