The authors used the Humphrey Retinal Analyzer to study the effect of acute changes in intraocular pressure (IOP) on the topography of the optic nerve head in normotensive and hypertensive eyes of cynomolgus monkeys. Chronically elevated IOP was produced monocularly in monkeys by argon laser angle treatment. In 8 hypertensive eyes, the mean IOP of 51 mmHg was lowered acutely to 15 mmHg with a needle placed in the anterior chamber. In 12 normotensive eyes, the mean IOP of 16 mmHg was similarly increased to 45 mmHg. Simultaneous stereophotographs were taken both before and within 15 minutes after the IOP change. The surface of normotensive optic nerve heads moved posteriorly a mean of 17.8 microns after IOP elevation (P less than 0.02). The surface of hypertensive optic nerve heads moved anteriorly 47.4 microns after IOP lowering (P = 0.1); this change was significantly less in nerve heads with larger and deeper cups (P less than 0.02). Significant changes were not detected in disc diameter or vein diameter with acute changes in IOP. Measurable shifts in the position of the optic nerve head surface when IOP is altered may provide a future prognostic test for glaucomatous optic neuropathy.