Primary events in the course of light induced retinal lesions are still not fully elucidated. Under chronic conditions, lipid peroxidation in the retina and death of photoreceptor cells are observed. The radioprotective agent WR-77913 scavenges singlet oxygen, hydrated electrons and free radicals. WR-77913 was used to protect against acute light induced photoreceptor outer segment membrane disruptions in the rat retina. There was a partial but not complete protection at higher illuminance levels (800 lx for 30 min), whereas threshold lesions (400 lx for 30 min) were almost completely prevented. These observations indicate an involvement of photodynamic reactions in causing acute photoreceptor lesions.