Anti-retroviral therapy promotes clinical, immunologic and virologic improvement in HIV-infected patients. Whereas this therapy adversely affects carbohydrate and lipid metabolism, the effects of anti-retroviral drugs on muscle protein synthesis and degradation have not been reported. To examine these processes, C2C12 myocytes were treated with increasing concentrations of the protease inhibitor indinavir for 1 or 2 days. Treatment of myocytes with a therapeutic concentration of indinavir (20 µM) for 24 h decreased basal protein synthesis by 18%, while a 42% decline was observed after 48 h. A similar decrement, albeit quantitatively smaller, was detected with other protease inhibitors. Indinavir did not alter the rate of proteolysis. Likewise, indinavir did not impair the anabolic effect of insulin-like growth factor-I on protein synthesis. Mechanistically, indinavir decreased the phosphorylation of the S6 ribosomal protein (rpS6), and this reduction was associated with a decreased phosphorylation of p70S6 kinase and p90rsk as well as the upstream regulators ERK1/2 and MEK1/2. Indinavir also decreased the phosphorylation of Mnk1 and its upstream effectors p38 MAPK and ERK1/2. Indinavir did not affect the phosphorylation of mTOR or 4E-BP1, but it did decrease the amount of the active eukaryotic initiation factor (eIF) 4G/eIF4E complex. In conclusion, indinavir decreased protein synthesis in myocytes. This decrease was associated with the disruption of the ERK1/2 and p38 MAPK pathways and a reduction in both the level of functional eIF4F complex and rpS6 phosphorylation.