Abstract The effects of 60 min exposure to high (50 mmHg) and low (0 mmHg) intraocular pressure in the outflow apparatus of the rhesus monkey have been assessed by transmission electron microscopy. At high pressure there is marked distension of the endothelial and outer corneoscleral parts of the meshwork with occlusion of the anterior part of Schlemm's canal, but in this region cellular integrity is maintained. In the posterior part of the meshwork, intercellular connections are disrupted and degenerative changes are observed in the trabeculae. At 0 mmHg intraocular pressure, the outer half of the meshwork is bowed inwards and compressed, and degeneration of the lining cells and trabecular cores is progressively advanced toward the inner meshwork. The graded preservation of the meshwork is attributed to diffusion of oxygen from the packed erythrocytes in the dilated canal of Schlemm.