Electric stimuli of constant but submaximal strength applied at constant intervals to the stump of the optic nerve of the rabbit after removal of the retina, result in general in electrical responses from the optic cortex which vary in amplitude. At certain definite frequencies however, 3-5 per sec. for different animals, responses are of constant height. As the stimulus is made stronger, the responses of the cortex become higher and of nearly constant amplitude, and occur earlier after the stimulus, and this specific frequency is not required. This is taken to signify that at some level of the pathway between retina and cortex a rhythmic variation in excitability takes place, such that with few fibers stimulated only impulses timed in exact phase with this rhythm pass with constant intensity. Since stronger stimuli activate more optic nerve fibers, the earlier arrival of the impulse at the cortex when more fibers are stimulated indicates facilitation between pathways, and the constant amplitude obtained with strong stimuli indicates that the cyclically recurring depression of excitability can finally be overcome by such facilitation. The fibers of lowest threshold lead to elements of the pathway that are normally in phase with each other, but are out of phase with elements whose optic nerve fibers are of higher threshold. A strong stimulus appears to initiate a new cycle, and to throw the different elements into the same phase, as tested by a second stimulus; that is, a rhythm which in independent of weak but effective stimulation can be driven by stronger stimuli.