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 (VO₂) in fed control and 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 that in the dark phase was unchanged; 2) the tail temperature fell from the control, greatly in the dark phase (P<0.05, 30.7 ± 0.1 to 23.9 ± 0.1 in the dark phase and 29.4 ± 0.1 to 25.2 ± 0.2°C in the light phase); 3) VO₂ decreased from the control (P<0.05, 24.37 ± 1.06 to 16.24 ± 0.69 ml/ min^.kg body weight ^0.75 in the dark phase and 18.91 ± 0.64 to 14.00 ± 0.41 ml/ min^.kg body weight ^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, the dark phase T_core was maintained by a suppression of 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 for the suppression of heat loss mechanism.