The changes of 10 enzymatic activities of cat retinas and their blood vessels under acute ocular hypertension were systematically observed by enzyme histochemical methods. These changes were induced by the damage caused by retinal ischemia-reperfusion. The activities of the free radical scavengers, catalase and hydrogen peroxidase, were decreased, demonstrating that the excess of free radicals is one of the essential causes of the injury. The principles of treatment are besides lowering of intraocular hypertension, during retinal ischemic stage, oxygen and nutrients needed urgently should be supplied through extra-vascular route and during reperfusion stage, free radical scavengers ought to be given.