Plasma glucose and muscle glycogen oxidation during prolonged exercise (75-min at 48 and 76% VO_2max) were measured in eight well trained male subjects (VO_2max = 4.50[0.63] L/min) using a simplified tracer technique in which a small amount of glucose highly enriched in ¹³C was ingested: plasma glucose oxidation was computed from ¹³C/¹²C in plasma glucose (which was stable beginning min 30 and 15 during exercise, at 48 and 76% VO_2max, respectively) and ¹³CO₂ production, and muscle glycogen oxidation was estimated by subtracting plasma glucose oxidation from total carbohydrate oxidation. Consistent data from the literature suggest that this small dose of exogenous glucose does not modify muscle glycogen oxidation, and has little effect, if any, on plasma glucose oxidation. The percent contributions of plasma glucose and muscle glycogen oxidation to the energy yield at 48% (15.1[3.8] and 45.9[5.8] %), and 76% VO_2max (15.4[3.6] and 59.8[9.2] %) were well in line with data previously reported for similar workloads and exercise durations, using conventional tracer techniques. The significant reduction in glycogen concentration measured from pre- and post-exercise vastus lateralis muscle biopsies paralleled muscle glycogen oxidation calculated using the tracer technique and was larger at 76% than 48% VO_2max. However, the correlation coefficients between these two estimates of muscle glycogen utilization were not different from zero at each of the two workloads. The simplified tracer technique used in the present experiment appears to be a valid alternative approach to the traditional tracer techniques for computing plasma glucose and muscle glycogen oxidation during prolonged exercise.