This study defines the tissue-specific changes in glucose metabolic flux that occur over time prior to the onset of whole-body insulin resistance in rats. Rats at 6 weeks of age were maintained on a high-carbohydrate diet for either 12 or 26 weeks, at which time euglycemic clamps were performed at basal and midphysiological plasma insulin concentrations. Following death, insulin-sensitive tissues were excised and frozen until assayed for the rate of glucose uptake, glycogenesis, and lipogenesis. Glucose metabolic flux, particularly through glycogenesis, was reduced between 18 and 32 weeks of age in all tissues except the adipose tissues. For example, the rate of glycogenesis in liver at 18 weeks (117+/-10 nmol glucose incorporated/min/g) was more than double that observed at 32 weeks (54+/-8 nmol glucose incorporated/min/g, P < .01). Despite this, animals in the 32-week group displayed no impairment in whole-body glucose disposal, due to compensatory glucose uptake in white adipose tissue (WAT) and increased glucose flux through lipogenesis in brown adipose tissue (BAT). At 32 weeks, the rate of glucose uptake in WAT (85.0+/-5.6 nmol 2-deoxy-D-glucose phosphate accumulated/min/g) was approximately double that at 18 weeks (46.6+/-5.6 nmol 2-deoxy-D-glucose phosphate accumulated/min/g) was approximately double that at 18 weeks (46.6+/-5.6 nmol 2-deoxy-D-glucose phosphate accumulated/min/g, P < .01). These changes in insulin responsiveness in adipose tissue of older animals may underlie the increased adiposity that is currently thought to be the causative factor in the development of age-related insulin resistance.