The alterations of the electroretinogram (ERG) induced by elevations of the intraocular pressure were not due to pressure alone but secondary to alterations of the blood flow into the eye because (1) consistent changes could be obtained only when the intraocular pressure was related to the blood pressure, (2) significant depression of the ERG did not occur until the intraocular pressure exceeded the diastolic pressure, and (3) the time course of the alterations was similar to that observed when the blood flow to the eye is blocked.