The relative cerebral cortical metabolism of glucose (GLU) and 2-deoxy-D-glucose (DG) was measured in vivo in control and insulin-treated hypoglycemic rats. The ratio of the utilization rate constants for the two hexoses, i.e., KDG/KGLU is defined as the Hexose Utilization Index (HUI). The HUI was found to be invariant in rats whose cerebral glucose content exceeded 1 mumol . g-1 wet weight (HUI = 0.48 +/- 0.07). Severe hypoglycemia (plasma glucose less than 2 mM) effected a shift in the HUI to 1.04 +/- 0.21. The results are consistent with a model in which the interpretation of the HUI is determined by the rate of transport into brain, or subsequent phosphorylation, as the rate-limiting step for hexose utilization.