Iodoantipyrine was used to record relative blood flow in the retina, choroid, optic nerve head, lamina cribrosa, and postlaminar optic nerve of cats at different levels of intraocular pressure. The IOP could be elevated to within 25 mm Hg of mean femoral arterial pressure, with only a slight effect on blood flow in the retina, choroid, and optic nerve head. At higher IOPs, the blood flow is reduced in the retina, choroid, and optic nerve head, but, in the lamina cribrosa, the blood flow is reduced only with extreme pressure elevation and is not reduced at all in the intraorbital optic nerve. Thus, there is no demonstrated effect of IOP on blood flow preferentially in the normal optic nerve. It is concluded that there is an efficient autoregulation in the optic nerve head and lamina cribrosa so that the IOP over a wide range does not much influence blood flow under normal circumstances. These findings do not rule out a role of ischemia in the pathophysiology of glaucomatous cupping, which may be caused by faulty autoregulation.