This paper reviews studies on the permeability of the blood-aqueous barrier to substances that are not transported by cellular mechanisms. The intercellular movement of small molecules and ions from the ciliary processes into the posterior chamber is restricted by tight junctions between the nonpigmented epithelial cells. For plasma proteins the endothelial cells of the capillaries constitute an additional barrier. The number of tight junctional strands and the electrical conductance of the epithelium is such that it can be classified as a border case between leaky and tight epithelia. The iris vessels seem to constitute a more efficient barrier in primates than in rabbits and cats most probably due to the presence of more complete junctional strands between the endothelial cells in primates. Although there is no anatomical barrier there is an efficient functional barrier between the stroma of the ciliary processes and the anterior chamber. The flow of aqueous humour most probably plays a role in the process of control of the barrier function in the routes between Schlemm's canal and the anterior chamber.