By contrast to adult, we have shown (Magne et al. 2011) that immobilization during ageing generated muscle atrophy which is not recovered. This is not due to a sustained muscle proteolysis since it was rapidly normalized. This study was undertaken to assess if an alteration of muscle protein synthesis during the immobilization and the recovery periods was involved and if a leucine supplementation may improve muscle mass following immobilization. 23-month-old rats were immobilized for 8 days (I8) and allowed to recover during 40 days. Half of the rats received a control diet whereas the other half were fed the same diet supplemented with 4.5% leucine after cast removal. Muscle proteasome-dependent proteolytic activities, FOXO3a-phosphorylation were assessed in gastrocnemius. Protein synthesis and protein S6 phosphorylation were measured at both the postabsorptive (PA) and post prandial (PP) states At I8, immobilized-muscles were atrophied by 21% that was explained by an increased proteasome activities (+20 to +80%) but also by a large decreased in protein synthesis at PP (-30%). During recovery, proteolysis and protein synthesis were normalized. With the leucine-diet, proteolysis was normalized earlier and protein synthesis was 30% higher in PP than in controls. These observations were correlated with an increased phospho-FOXO3a/FOXO3a ratio (+80%) and a sustained increase of phospho-S6 amount (+30%) However, despite this improvement of muscle anabolism, leucine failed to improve muscle mass recovery. After immobilization, leucine supplementation, despite its beneficial effect on muscle protein anabolism failed in muscle mass recovery. This discrepancy needs to be further studied.