PURPOSE: To understand how hydrodynamic and morphologic changes in the aqueous humor outflow pathway contribute to decreased aqueous humor outflow facility following acute elevation of intraocular pressure (IOP) in bovine eyes. METHODS: Enucleated bovine eyes were perfused at one of four different pressures (7, 15, 30 and 45 mmHg), while outflow facility was continuously recorded. Dulbecco's PBS +5.5 mM glucose containing fluorescent microspheres (0.5 microm, 0.002% v/v) was perfused to outline aqueous outflow patterns followed by perfusion-fixation. Confocal images were taken along the inner wall (IW) of the aqueous plexus (AP) in radial and frontal sections. Percent effective filtration length (PEFL= IW length exhibiting tracer labeling/total length of IW) was measured. Herniations of IW into collector channel (CC) ostia were examined and graded for each eye by light microscopy. RESULTS: Increasing IOP from 7 to 45 mmHg coincided with a 2-fold decrease in outflow facility (p < 0.0001), a 33-57% decrease in PEFL with tracer more confined to the vicinity of CC ostia, progressive collapse of the AP and increasing percentage of CC ostia exhibiting herniations (from 15.6+/- 6.5% at 7 mmHg to 95+/-2.3% at 30 mmHg (p < 10(-4)), reaching 100% at 45 mmHg). CONCLUSIONS: Decreasing outflow facility during acute IOP elevation coincides with a reduction in available area for aqueous humor outflow and confinement of outflow to the vicinity of CC ostia. These hydrodynamic changes are likely driven by morphologic changes associated with AP collapse and herniation of IW of AP into CC ostia.