Nine eyes of five cynomolgus monkeys were perfused through the anterior chamber with cationized ferritin (CF) at normal and increased intraocular pressure. After perfusion with glutaraldehyde, the morphologic appearance of the endothelial lining of Schlemm's canal (SC) and of the adjacent regions was analyzed using tangential and sagittal serial sections. The CF, which binds to negatively charged surfaces, was observed to be adherent to the free surfaces of trabecular cell membranes and to accumulate in the cribriform layer underlining the endothelial lining of SC. Tangential sections of the inner-wall endothelium demonstrated that separations of the adjacent cell membranes occur between the tight junctions forming openings or lacunae and bent, tunnel-like channels that represent continuous paracellular pathways. Complete staining of these inner-wall paracellular pathways with CF were observed indicating that the adjoining membranes are negatively charged and that the perfused fluid had passed through these intercellular channels. These paracellular pathways appeared enlarged and were more easily identified at elevated perfusion pressure. In general, intracytoplasmic vacuoles demonstrated heavy staining with CF on their luminal surface but only faint staining on the adluminal (cribriform-facing) surface. Apparent giant vacuoles were observed to be often not real intracellular vacuoles but rather dilatations of the paracellular spaces. This study demonstrates that there are paracellular routes through the inner-wall endothelium by which high molecular-weight substances such as ferritin and macrophages can leave the anterior chamber. Probably there are both transcytoplasmic and paracellular mechanisms of aqueous outflow that may vary under different conditions of pressure or flow.