The effects of training on leucine turnover and oxidation were studied during rest and exercise in young, postabsorptive female rats. Relationships were primarily studied by the continuous infusion method, using [1-14C]leucine. Arterial blood was assayed for leucine specific activity and expired air was monitored for O2 consumption, CO2 production, and the specific activity of 14CO2. The rate of leucine turnover in trained rats was 70% greater than the rate of leucine turnover in untrained rats and was not affected by exercise. By virtue of a higher turnover, the rate of leucine oxidation was 40% greater in trained rats during both rest and exercise. Moreover, leucine oxidation was proportional to the metabolic rate and represented 30% of the leucine turnover at rest and 40-60% of the leucine turnover during exercise. In the same animals leucine turnover was secondarily assessed by measuring the fraction of the [1-14C]leucine dose contained in the hemoglobin pool 4 wk after infusion (occupancy tracer method). Occupancy values for leucine turnover were not different from those determined by the continuous infusion method. The results show that whole body leucine turnover is increased by training and that leucine oxidation is increased by both training and exercise.