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.