Body temperature was measured telemetrically during day and night on unrestrained Syrian hamsters. The variation in temperature was taken as an indirect measure of the locomotor activity of the animals. A significant circadian temperature rhythm characteristic for nocturnal rodents was only found when the hamsters were caged together in a group. When isolated the mean body temperature during both the light and the dark period decreased and the circadian temperature rhythm was damped. The differences in the temperature rhythm in different rodents is stressed. The attention is called to the fact that the circadian temperature rhythm can be used in estimation of the optimum conditions for the experimental animals in order to get the most homogeneous material for the experiment.