The effect of unilateral optic nerve section on the susceptibility of rat photoreceptors to damage by constant light was studied. Optic nerves of albino rats were cut intracranially by a ventral approach, so as not to interfere with structures in the orbit of the eye, with the brain or with the blood supply to the retina. Two separate experiments were performed on unilaterally optic nerve sectioned rats from two different sources. One group was purchased from a commercial supplier and the other group was born and raised in 3 lux cyclic light in our laboratory. For 1-4 weeks after surgery they were exposed to either 1000 lux for 24 hr or 80 lux for 48 hr. Light damage was quantified by measuring the outer nuclear layer area remaining in histological sections through the vertical meridian of the retina. It was found that retinas with optic nerves cut suffered substantially less damage from light than did those with intact optic nerves. Sham operated rats suffered the same amount of damage as did the optic nerve intact retinas of rats with one nerve cut. The extent of protection was greatest in the region previously shown to be most susceptible to damage. The protection afforded by optic nerve section could not be explained on the basis of behavior or rhodopsin photochemistry. The possible role of heat-shock proteins and a neuromodulator is discussed.