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Eight rabbits were synchronized by LD 14:10 (200-250: 0 Lux). After more than 4 weeks of exposure to this Zeitgeber two types of experiment were performed on each specimen. In type I experiments the electroretinograms (ERG's) and Visually Evoked Potentials in response to 3 μsec flashes were recorded during the "day-time-potential" phase of the circadian rhythm, at first during steady illumination at either 1200 or 2500 Lux and thereupon in darkness, and also during the "night-time-potential" phase, but then firstly in darkness and afterwards during illumination with the same intensity. In type II experiments the responses to the same flashes were, again, recorded in both phases but now the sequence of illumination -at identical leveland darkness was reversed. It appeared that programmed changes in photic responses occurred both in darkness (in type I experiments) and during steady illumination (in experiments of the other type) but that the circadian change in amplitude of the ERG b waves was larger for the ERG's recorded in darkness than for those obtained during illumination. It further appeared that the suppressive influence exerted by background illumination on the ERG's is stronger during the N.T.P.-phase of the rhythm than in the other phase.The possible roles of this circadian rhythm in retinal photosensitivity for the daily survival of rabbits, and in the origin of photoperiodic changes, are discussed.

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.