Repeated blood sampling in rodents is often necessary and difficult. Magnesium has been touted as an agent for enhancing physical activity. An auto-blood-sampling device coupled with a microdialysis analyzer was developed to determine blood glucose and lactate concentrations in rats subjected to treadmill exercise. The effects of magnesium on exercise performance and blood energy metabolism were also evaluated. Sprague-Dawley rats fed a magnesium-adequate diet were randomly assigned to 2 experimental groups. Exercise performance was evaluated at 3 treadmill speeds (10, 15, and 20 m.min(-1)) with or without magnesium administration (90 mg.kg-1, intraperitoneal) in the first experiment. In the other experiment, each rat was fitted with a catheter in the jugular vein for collection of blood samples during the treadmill exercise at a speed of 20 m.min(-1). Exercise performance was significantly higher at the lower speed of 10 m.min(-1) in the control group. In addition, exercise performance was significantly enhanced only at 20 m.min(-1) in the magnesium-sulfate-treated group when compared with the control group. Blood samples were collected every 15 min. Glucose concentrations increased significantly and then declined immediately after completion of the exercise task at 20 m.min-1 in both groups. However, glucose concentrations increased immediately after administration of magnesium and increased further during exercise when compared with those of the control group. Findings from a repeated blood-sampling assay suggest that increased blood glucose contributes to enhanced exercise performance by rats injected intraperitoneally with magnesium.