The flux of glucose at the anterior stromal surface of the cornea is 575 nanomol/sq cm/hr in anesthetized rabbits. Blockade of the endothelial surface with air and silicone oil reduces the glucose flux to less than 20% of normal, reflecting the small amount of glucose which is available to the epithelium from the limbus. Blood in the anterior chamber reduces the flux to about 50% of normal. Interference with normal endothelial function by mechanical or thermal damage increases the movement of glucose across the cornea, which may indicate that the endothelium ordinarily acts as a relative permeability barrier to glucose penetration from the aqueous humor into the corneal stroma.