We induced ischemia, hypertension and hypotension in 15 rabbits in order to evaluate the ischemic changes in the optic nerve and the effect of hypertension and hypotension on ischemia. We cauterized the right internal and external carotid arteries of 15 rabbits and applied dopamine hydrochloride and glycerol trinitrate to 5 each of these rabbits. Two rabbits were used as controls, We enucleated both eyes of all animals at the 24th hour. All of the optic nerves underwent biochemical, histopathological and ultrastructural examination. Histopathological and transmission electron-microscopic changes were found to be more prominent in the hypotensive group, We observed decreased superoxide dismutase levels in all groups, but it was more evident in the third group, In comparison to hypertension, hypotension is found to be a more important factor in the development of early degenerative changes. Copyright (C) 2001 S. Karger AG.,Basel.