We performed pressure on the left optic nerves of 24 pigs and injected intravenous thyroid-releasing hormone (TRH) to 12 of these pigs in order to evaluate the degeneration and regeneration in the optic nerves. The histopathologic, ultrastructural and biochemical examinations of the optic nerves were made at the 24th hour, 7th, 15th and 30th days. although the light-microscopic examinations were normal, ultrastructural changes of the uncompressed optic nerves were interesting. Histopathologic and ultrastructural investigation of the compressed optic nerves showed significant degenerative changes in the non-TRH-applied group. Ultrastructural comparison yielded lighter degenerative changes in the TRH-applied group but there was no clue showing the stimulation of regeneration. We observed increased superoxide dismutase (SOD) levels in the nontraumatized optic nerves due to the cellular stress. The SOD values were found to be low in highly damaged left compressed optic nerves indicating the prevention of SOD enzyme synthesis.