Deep body temperature and locomotor activity of rats fed a reduced food amount (n = 9) and of starved rats (n = 9), were measured by implanted transmitters. Both groups were then refed ad libitum. The reduction in body temperature was significant for both groups, but larger in the starved rats than in the food restricted rats. There was a displacement of the circadian temperature rhythm in the starved rats. There were no significant differences in locomotor activity between groups until the start of refeeding. Rats fed reduced food amounts rapidly increased their activity, while the starved group retained a low activity for several days. Thermal conductance was reduced by 30% in both groups. This reduction in thermal conductance may explain how starving and semistarving rats are able to maintain core temperatures close to normal, even if resting metabolic rates are drastically reduced. The measured reduction in body core temperature signifies a change in the thermoregulatory 'set-point' during starvation.