Arteriovenous difference and tracer dilution techniques were utilized to determine the effect of exercise on whole body, gut, liver, and splanchnic leucine kinetics. Five postabsorptive dogs were infused with [1-13C]leucine and studied during rest, 90 min of moderate-intensity treadmill exercise (1st 45 min, early; last 45 min, late exercise), and 90 min of recovery. The whole body leucine rate of appearance (Rai; mumol.min 1.kg-1) increased from rest (3.33 +/- 0.11) during early (3.68 +/- 0.14) and late (4.24 +/- 0.27, P < 0.05) exercise and was 3.41 +/- 0.19 during recovery. Gut Ra increased from rest (0.64 +/- 0.08) during early (0.92 +/- 0.12) and late (1.30 +/- 0.20, P < 0.05) exercise and was 0.77 +/- 0.16 during recovery. Liver leucine Ra did not significantly change (P > 0.05). The whole body leucine rate of disappearance (Rd) paralleled whole body leucine Ra throughout. Leucine Rd across the gut, liver, and splanchnic bed, however, did not significantly change (P > 0.05), indicating an increase in leucine uptake outside of these regions. Because active skeletal muscle is likely the principal consumer of these amino acids, the data suggest that gut protein-derived amino acids are utilized for the attenuation of net muscle protein catabolism during and immediately following exercise.