MafBx and Murf are two new rat E3 ubiquitin ligases induced in muscle atrophy. Our goal was to investigate whether lipopolysaccharide (LPS) injection, a model of muscle catabolism, is associated with increased expression of MafBx and Murf. LPS (750 W Wg/100 g body weight) induces MafBx and Murf mRNA (respectively, 23-fold and 33-fold after 12 h; P 6 0.001). A transient induction of tumor necrosis factor-K K mRNA (21-fold; P 6 0.001 at 3 h) and a decrease of insulin like growth factor-I mRNA (50%; P 6 0.001 at 6 h), two potential regulators of the ubiquitin^proteasome system were also demonstrated. In summary, MafBx and Murf mRNA are upregulated in response to LPS and might play a role in the muscle proteolysis observed. ß