Amino acid metabolism and proteolysis in rat skeletal muscle following chronic exercise. Rats were trained on a motor-driven treadmill for 16 wk. The soleus, red vastus lateralis, and white vastus lateralis muscles were assayed for myofibrillar, lysosomal, and soluble protein content and for the degradation of the radioactive tracer [14C] leucine. Total protein content increased in both fast-twitch red (red VL) and slow-twitch soleus (SOL) muscles (20-25%) following exercise training. Myofibrillar protein showed a small (7%) increase in red VL and a large (70%) increase in the SOL. Myofibrillar and soluble protein content were unchanged in the white vastus lateralis (WV). Lysosomal protein did not change in red VL, WV, or SOL. Degradation of [14C] leucine was accelerated only in the SOL (10%), while the rate of proteolysis (the ratio of degradation to protein synthesis) increased by 46% in red VL, WV, and SOL, and by 20% in white VL. We conclude that: 1) the increase in protein content is greatest in the SOL, the slow-twitch, mixed-fiber muscle, while the increase in proteolytic capacity is greatest in fast-twitch muscles; 2) changes in lysosomal and myofibrillar proteins, including those that have a greater turnover, did not show a statistically significant response; and 3) increased degradation of myofibrillar and soluble proteins was offset by a decrease in the rate of protein synthesis in both red VL and SOL muscles. We suggest that red VL and SOL muscles respond to exercise by the preferential formation of new protein in the form of myofibrils, thus enhancing their size and contractile capacity.