A non-invasive biochemical technique for quantifying the effects of anterior corneal hypoxia on the in vivo corneal epithelium of the human eye is described. Following short-term exposure of the cornea to low atmospheric oxygen pressures, lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) activities in tears are altered so that the tear LDH/MDH ratio is elevated. The degree of elevation of the ratio and its timing are related to the severity of hypoxia. Possible explanations for the elevation of the tear LDH/MDH ratio include unbinding of intracellular LDH and increased cell membrane permeability. For severe hypoxia, de novo LDH synthesis may also contribute. These changes suggest that control mechanisms within the corneal epithelium may be responding to optimize the efficiency of anaerobic metabolism during conditions of environmental stress, and offer the prospect of a non-invasive technique for monitoring epithelial metabolic stress.