It was previously established that when rabbits have been exposed to 24‐h LD alternations for several weeks, they afterwards exhibit in steady darkness the presence of a programmed rhythm in photic responses of the visual system. In one phase of this rhythm they respond to flashes with “Day Time Potentials”; (D.T.P.’s) while the other phase is characterized by a markedly raised retinal photic sensitivity and a consequent responding with “Night Time Potentials”; (N.T.P.’s). It is also known that these changes of the responses,e.g.of the ERG's , are absent after transection of both cervical sympathetic nerves whereas they can easily be elicited by low‐frequency electrical stimulation of these nerves. From the present experiments it appears that similar D.T.P. ‐ N.T.P. conversions of the ERG's can be induced by highly localized electrical stimuli that are applied at rates of 10 cps or less to either the suprachiasmatic nuclei (SCN's) or to a tract descending caudally from these towards the spinal cord. Such effects of SCN stimulation are absent in rabbits in which the cervical sympathetics have been cut beforehand.