The effects of acute intraocular pressure (IOP) on the reduction/oxidation ratio of cytochrome a, a 3 were measured from intact cat optic nerve by microfiber reflection spectrophotometry. This enabled the real-time analysis of optic nerve-head oxidative metabolism following IOP or mean arterial pressure (MAP) changes. Findings included: (1) cytochrome a, a 3 became more reduced and relative blood volume decreased at lower perfusion pressures, even at IOP of <20 mm Hg; (2) metabolic inhibition began at variable perfusion pressures but invariably progressed as perfusion pressure declined; and (3) increased IOP or decreased MAP caused metabolic inhibition. These findings demonstrate that: (1) optic nerve metabolic dysfunction is possible at low IOPs; (2) lowering IOP can reverse metabolic dysfunction; (3) the metabolic response is dependent on IOP and/or MAP changes; and (4) the metabolic inhibition is related to optic nerve ischemia. © 1990 Springer-Verlag.