We have investigated the effects of changes in brain temperature on the electroencephalogram (EEG) during entrance into daily torpor, a natural hypothermic state, in the Djungarian hamster. A systematic shift of single EEG frequencies was found as cortical temperature decreased. The relation between EEG frequency and cortical temperature was very similar to the temperature dependence of the Na(+)-K(+)-pump, suggesting that the pump is the rate-limiting step in determining EEG frequency.