We examined the intracellular metabolic fate of plasma glucose during a hyperglycemic clamp in impaired glucose tolerant (IGT; N=21) and normal glucose tolerant subjects (N=10) using a combination of [3-³H] glucose infusion with measurement of [³H] water formation and indirect calorimetry. IGT was associated with ~35% reduced first phase insulin responses, normal second phase insulin response and 25-30% reduced insulin sensitivity, resulting in ~35% reduced plasma glucose disposal. This was coupled with ~55% reduced storage of plasma glucose (P<0.01) and ~15-20% reduced glycolysis of plasma glucose (P<0.03), accounting for ~75 and 25% of the reduction in glucose disposal, respectively. Decreased glucose oxidation accounted for virtually all the decrease in glycolysis. Therefore, non-oxidative glycolysis of plasma glucose in IGT was similar to that in NGT (P>0.9) and accounted for an increased proportion of systemic glucose disposal (p<0.05). We conclude that in IGT, decreased disposal of plasma glucose mainly involves decreased glycogen synthesis and to a lesser extent decreased glycolysis, which is accounted for by decreased glucose oxidation. An increased proportion of plasma glucose hence undergoes non-oxidative glycolysis, representing a novel early abnormality in the pathogenesis of T2DM.