The hypothalamic temperature (Thy) and the rate of oxygen consumption of golden-mantled ground squirrels were continuously measured as they entered hibernation. Thy was manipulated with chronically implanted, water-perfused thermodes. A threshold Thy for eliciting an increase in metabolic heat production was demonstrable at all times during entry. During smooth entries this threshold Thy showed a progressive decline so that it was below actual Thy at all times. These results are interpreted to mean that the normal mammalian central nervous regulator of body temperature is functional throughout the entrance into hibernation, and it can be reset to any level over the 35 degrees C range body temperatures experienced by the hibernator.