The existence of a secondary aqueous humor outflow pathway was studied in eyes of rabbits, cats, and monkeys by following the movement of fluorescein, vinyl particles, a protein dye complex, and inulin labeled with radioactive carbon (14C). Following an intracameral injection fluorescein rapidly appeared in the suprachoroidal space in the eyes of all three species. The large particles 1μ to 3μ in diameter moved from the anterior chamber to the suprachoroidal space in the monkey eyes; they only moved as far posteriorly as the ciliary body in the cat, and they did not penetrate either this tissue or the iris in the rabbit. The smaller colloidal particles of the protein dye complex left the anterior chamber of the rabbit eye, entered the iris, ciliary body, and cleft and finally appeared in the choroidal vessels and vortex veins. Determinations with inulin 14C indicated that approximately 8% of the aqueous humor left the normal rabbit eye through the vortex veins. This percentage was slightly higher in the buphthalmic rabbit eyes.