To investigate the mechanism involved in the reduction of body core temperature (T_core) during fasting in rats, which is selective in the light phase, we measured T_core, surface temperature, and oxygen consumption rate in fed control animals and in fasted animals on day 3 of fasting and day 4 of recovery at an ambient temperature (T_a) of 23°C by biotelemetry, infrared thermography, and indirect calorimetry, respectively. On the fasting day, 1) T_core in the light phase decreased (P < 0.05) from the control; however, T_core in the dark phase was unchanged, 2) tail temperature fell from the control (P < 0.05, from 30.7 ± 0.1 to 23.9 ± 0.1°C in the dark phase and from 29.4 ± 0.1 to 25.2 ± 0.2°C in the light phase), 3) oxygen consumption rate decreased from the control (P < 0.05, from 24.37 ± 1.06 to 16.24 ± 0.69 ml · min¹ · kg body wt^0.75 in the dark phase and from 18.91 ± 0.64 to 14.00 ± 0.41 ml · min¹ · kg body wt^0.75 in the light phase). All these values returned to the control levels on the recovery day. The results suggest that, in the fasting condition, T_core in the dark phase was maintained by suppression of the heat loss mechanism, despite the reduction of metabolic heat production. In contrast, the response was weakened in the light phase, decreasing T_core greatly. Moreover, the change in the regulation of tail blood flow was a likely mechanism to suppress heat loss.