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Recent reports suggest that the aphakic eye develops less corneal edema than the phakic eye during hydrogel contact lens wear. Measurements were made of several ocular characteristics that might account for this difference in nine unilateral aphakic subjects. The measurements showed that compared with the phakic eye, the aphakic eye averaged a 15% lower epithelial oxygen uptake rate, 18% fewer endothelial cells, 85% loss of corneal sensitivity, and 8% thinner corneal epithelium. Both eyes of each subject were exposed to a reduced oxygen environment for 2 hr in separate experiments: (1) by wearing a thick hydrogel contact lens and closing the eyes, and (2) by exposing the eye to 100% nitrogen gas. In the gel lens-closed eye experiment the aphakic cornea swelled an average of 7.5%, compared with 11.0% for the phakic cornea. For the nitrogen gas experiment the results were 4.3% and 6.2%, respectively. Eighty-five percent of the edema was in the stromal layer. The lower swelling response for the aphakic eye was significant (p less than 0.01) in both experiments. Corneal swelling after hypoxia is considered to be caused by an increase in corneal osmolarity resulting from an increase in the rate of production of lactic acid. The reduced corneal swelling in the aphakic eye, when compared with the phakic eye, could therefore be the result of either (1) a reduction in the overall metabolic activity of the epithelium, as indicated by the lower epithelial oxygen uptake rate and the thinner corneal epithelium in the aphakic eye, or (2) an increased efflux of lactic acid from the cornea resulting from an increased endothelial permeability to lactate.