Corneal thickness changes were measured on human subjects who wore gel lenses that varied in center thickness. Using these measurements and the results of an earlier study in which changes in corneal thickness were monitored on human corneas exposed to oxygen tensions below that in air, we showed that the oxygen tension under most contact lenses varies from 0 to 25 mm Hg which produces a corresponding oxygen flux into the cornea of 0 to 6 microliter cm2 hr. A critical oxygen tension and flux under the lens was found to be 10 mm Hg and 2 microliter cm2 hr, respectively, below which corneal swelling occurs. To maintain these critical levels of tension and flux, the minimum oxygen transmissibility of a stationary lens on the cornea was determined to be 5 X 10(-9) and 15 X 10(-9) (cm X ml O2)/(sec X ml X mm Hg) for the open and closed eye conditions, respectively.