The outflow apparatus from adult baboon and rabbit eyes was stained with the inorganic dye ruthenium red. The ruthenium reaction product coated the surface of the trabecular meshwork cells and the canalicular endothelial cells. Deposits also impregnated the various connective tissue elements within the trabeculae and the extracellular spaces of the endothelial meshwork. A fine fibrillar network could also be identified with ruthenium red and this was present in the trabecular cores and the extracellular spaces of the endothelial meshwork. It was considered that the fibrillar network may represent a matrix of glycosaminoglycans and glycoproteins. The significance of these materials in relation to aqueous outflow was discussed.