It has been shown that illumination of rod outer segment suspension in the presence of photosensitizers (methylene blue λ ≥ 620 run; retinal 370 ≥ λ ≥ 390 nm) results in chemical modification of the lipid and protein components of the photoreceptor membranes. This modification can be registered by accumulation of lipid peroxidation (LPO) products as well as oligomerisation of rhodopsin and a decrease of rhodopsin thermal stability. These effects are prevented by ′02-quenchers and free radical scavengers. It has been found that the electric activity (ERG) of isolated frog retina is inhibited due to photosensitized generation of ′02 which can be overcome by preliminary addition of ′03-quenchers and free radical scavengers to the incubation medium. The LPO products are accumulated in the retinae of rats exposed to high intensity light in vivo. It is concluded that ′02 and LPO are involved in light-induced damage of the retina.