Body temperature (T_b) of rat pups (7-9 days old) raised under a 12:12-h light-dark (L-D) regimen (L: 0700-1900, D: 1900-0700) was consistently higher in D than in L by ~1.1°C. We tested the hypothesis that the L-D differences in T_b were accompanied by differences in the set point of thermoregulation. Measurements were performed on rat pups at 7-9 days after birth. O₂ consumption (O₂) and CO₂ production (CO₂) were measured with an open-flow method during air breathing, as ambient temperature (T_a) was decreased from 40 to 15°C at the constant rate of 0.5°C/min. At T_a 33°C, O₂ was not significantly different between L and D, whereas CO₂ was higher in L, suggesting a greater ventilation. Over the 33 to 15°C range the O₂ values in D exceeded those in L by ~30%. Specifically, the difference was contributed by differences in thermogenesis at T_a = 30 to 20°C. As T_a was decreased, the critical temperature at which O₂ began to rise was lower in L. We conclude that the higher T_b of rat pups in D is accompanied by a higher set point for thermoregulation and a greater thermogenesis. These results are consistent with the idea that, in newborns, endogenous changes in the set point of thermoregulation contribute to the circadian oscillations of T_b.