A 48-h starvation period resulted in a great increase in muscle proteolysis-as measured following the release of tyrosine into the medium-in incubated isolated rat extensor digitorum longus (EDL) muscles. We have quantified the contribution of the different proteolytic systems to the increased protein degradation and observed a considerable activation in the ATP-dependent proteolytic (60%) and in the calcium-dependent (125%) systems, while no increases were observed in lysosomal proteolysis. The addition of 10 mM leucine to the incubation medium did not result in any changes in either total proteolytic rate or the activity rates of any of the different systems studied. In addition, the presence of the amino acid did not influence the levels of mRNA for the different genes studied-ubiquitin, C8 proteasome subunit, E2 conjugating enzyme, m-calpain, and cathepsin B. In a similar way, as observed during starvation, tumor growth resulted in increased protein degradation in incubated isolated EDL muscles from animals bearing the Yoshida AH-130 ascites hepatoma. The increased rate of protein degradation affected all the proteolytic systems studied: ATP- and calcium-dependent and lysosomal. Finally, leucine addition (10 mM), although not able to revert the increased proteolytic rate, resulted in a decrease in the gene expression for ubiquitin, C8 proteasome subunit and cathepsin B.