The cornea normally receives its oxygen from both the atmosphere and the aqueous humor. When a thick polymethylmethacrylate contact lens is placed on the cat cornea, access of atmospheric oxygen to the cornea is restricted, and the outer part of the cornea becomes hypoxic. This results in an increase in the oxygen flux from the aqueous humor into the cornea, and the oxygen tension in the aqueous humor decreases, as is demonstrated in this study. This increased oxygen flux from the aqueous into the cornea tends to alleviate the hypoxia caused by a corneal contact lens. Thus, the cornea can tolerate a thicker contact lens with less hypoxia than would be expected if only the oxygen supply from the atmosphere were considered.