The thermolability of lens hexokinase is implicated in the development of an experimental hypoglycemic cataract. After 8 h of glucose deprivation, there was a precipitous loss of lens hexokinase. This occurred approximately 9 h prior to the disorganization of the other enzymatic steps in glycolysis. Epithelial hexokinase, as an immediate response to glucose deficiency, shifted from the soluble to the insoluble phase. There was no such shift in the cortex-nucleus where only soluble hexokinase was found. After 8 h of glucose deprivation, both soluble and insoluble hexokinases throughout the lens underwent rapid deactivation. During the first 8 h of glucose deprivation the loss of lenticular ATP and K+ and the gain in wet weight could be reversed by restoring normal glucose levels; beyond 8 h the changes were irreversible. During the period of reversibility, hexokinase activity levels were normal; during the period of irreversibility hexokinase activity was 10-20% of normal. Of the substances tested (mannose, galactose, fructose, glutamine, adenosine) only mannose could sustain the lens in the absence of glucose. Neither endogenous free glucose nor glycogen could sustain the lens in the face of glucose deprivation. There appeared to be no alternative exogenous or endogenous energy yielding substrates. The younger the animal, the more susceptible is its lens to glucose deprivation. This most certainly is a reflection of the increased susceptibility of younger lenses to osmotic stress, since lenses in each age group manifested similar changes in hexokinase activity, ATP, Na+ and K+ level.