Enucleated eyes were perfused alternately via the anterior and vitreous chambers. At low intraocular pressure (IOP), vitreous humor presented considerable resistance to forward flow of perfusion fluid in calf eyes, but not in human eyes. In human eyes when the perfusion pressure was increased to 60 mm Hg, the resistance to flow forward from the vitreous body increased, but became practically nil again when the IOP was decreased. At high pressure the volume of the vitreous body apparently increases and the anterior hyaloid membrane probably presses against the ciliary body, reducing the area of hyaloid membrane through which fluid can flow. Whether increased perfusion pressure can in some other manner change the permeability of human vitreous to resemble that of the calf remains unanswered. Our results suggest that factors other than, or in addition to, simple diversion of aqeous humor must be important in malignant glaucoma.