The mechanisms of accelerated skeletal muscle protein degradation during sepsis have not been fully elucidated. Activity of the lysosomal protease cathepsin B is increased in skeletal muscle during various catabolic states other than sepsis. In the present study the protein degradation rate and cathepsin B activity were determined in extensor digitorum longus and soleus muscles from nonseptic and septic rats. The protein degradation rate during incubation in vitro with or without the cathepsin B inhibitor leupeptin was also determined. Both protein degradation and cathepsin B activity were increased in muscles from septic rats. Incubation with leupeptin reduced, but did not normalize, the protein degradation rate in both extensor digitorum longus and soleus muscles from septic animals. These studies suggest that increased cathepsin B activity contributes to the accelerated muscle proteolysis seen during sepsis and that proteases other than cathepsin B are also involved.