Control and endurance-trained rats received continuous infusions via jugular catheters of [U-14C]- and [6-3H]glucose under one of three conditions: rest (Re), running at 13.4 m/min (easy exercise, EE), or running at 26.8 m/min (hard exercise, HE). Arterial blood was sampled from carotid catheters. Blood glucose levels were not different between groups at rest (3.88 +/- 0.19 mM) or EE (4.32 +/- 0.35 mM). During HE, trained animals maintained blood glucose better (3.41 +/- 0.34 mM) than did untrained animals (3.03 +/- 0.42 mM). Respiratory exchange ratio (R) increased from rest (0.79 +/- 0.05) to exercise and was significantly lower in trained than in untrained animals during HE (0.87 +/- 0.02 vs. 0.93 +/- 0.03). Glucose turnover (Rt) calculated from [3H]glucose was not different between groups at rest (46.2 +/- 2.7 mumol x kg-1 x min-1). Turnover increased during EE to 91.5 +/- 7.5 vs. 72 +/- 8.5 mumol x kg-1 x min-1 in untrained and trained animals, respectively. During HE, Rt rose to 95.0 +/- 12.6 in trained animals but fell to 78.7 +/- 9.9 mumol x kg-1 x min-1 in untrained animals. The percentage of glucose flux oxidized increased from rest (44.0 +/- 6.8%) to exercise and was significantly lower in trained (73.7 +/- 4.3%) than in untrained animals (95.1 +/- 3.8%) during HE. Metabolic clearance rate increased from 12.5 +/- 0.8 in Re to 29.4 +/- 6.0 ml x min-1 x kg-1 in HE but did not differ between groups. Training improved glucose homeostasis during HE by increasing the glucose flux and by reducing the fraction of the flux lost to oxidation.