Ten adult rabbits were exposed to a LD 12:12 regimen for 18 months. They were initially intact, then deprived of their cervical sympathetic trunks and, finally, also blinded. In each condition experiments were performed, lasting 54 h in darkness, that showed in intact and sympathectomized animals the presence of a programmed rhythm in root‐mean‐square value (σ) of the occipital EEG. Sympathectomy reduced the amplitude of this rhythm by one half, whereas it raised the levels forgof the EEG's and EMG's by some 7% and induced a slight alteration of the EEG spectrum in favour of the frequencies 0.5–4.5 cps. When the optic nerves had been sectioned also, the rhythm was free‐running while the g ‘s of both the EEG and EMG had been further increased by more than 20% and the EEG spectrum had markedly shifted in favour of, again, the lower frequencies. The results point to an important role for the retinal dark discharge and suggest that the latter is modulated by the central circadian system by way of the cervical sympathetic nerves.