Rabbits that have been exposed for several weeks to a L12:D12 regimen exhibit afterwards, under constant circumstances and in steady darkness for 53 h, the presence of “programmed”; circadian rhythms in the RMS value and spectral composition of their (computer‐analysed) EEG's, in the RMS value of their electromyograms, and in skin temperature. For the occipital and frontal EEG's the RMS value attains its lowest level during a dip occurring about 3 h before the previous light on‐moment and its maximum, which is 1.5 times the minimal value, during a peak at some 3 h before the previously occurring light off‐transient. Similar changes occur in the power spectral densities of all frequencies between 0.5 cps and 40 cps, with the result that circadian changes in the spectral composition of the EEG, although present and having the same time course, are rather slight. The results are discussed in terms of the two‐oscillator model for the central circadian mechanism.