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.

It was previously shown that rabbits which have been exposed for several weeks to the natural sequence of daylight and nocturnal darkness, or to fixed 24-h L:D alternations, exhibit afterwards a programmed rhythm in retinal sensitivity to flashes and steady illumination. This rhythm consists of a phase in which the animals steadily respond to flashes with "Day Time Potentials" (D.T.P.'s) for the ERG's and cortical responses, and a phase with "Night Time Potentials" (N.T.P.'s) connected with raised photic sensitivity. The rhythm is obliterated by bilateral sectioning of the cervical sympathetic nerve (CSN), after which the responses are constantly of the D.T.P.-type. It now appears, from a comparison between the programmed fluctuations in the ERG's and cortical V.E.P.'s and their induced changes during low-frequency stimulation of the CSN's, that the latter contain fibers which discharge rarely, if at all, in the D.T.P.-phase and fire at the low rates of 2 cps or less during the N.T.P.-phase -and evoke in this way the circadian rhythm in photic sensitivity. It would also appear that the noradrenaline released by the postganglionic fibers in question acts upon alpha receptors which are probably situated in the inner nuclear layer of the retina. These results give further evidence for the presence in rabbits of a retino-hypothalamo-retinal loop with a 24 hours-delayed feedback.

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