The rate of oxygen uptake by the cornea from the membrane of a polarographic oxygen sensor was measured when there was a sudden change in the known oxygen tension at the anterior corneal surface. This procedure gave a set of standard curves of oxygen uptake rate versus time after change in oxygen tension for each initial oxygen tension at the corneal surface. The oxygen uptake rate observed in the same manner when a contact lens was suddenly removed from the cornea could be compared to the standard curves to estimate the oxygen tension under the lens while it was being worn. By this method it was shown that under a tight fitting contact lens the oxygen tension is close to zero whereas under a lens properly fitted by today's clinical criteria the oxygen tension is about 20 mm Hg.