We sought to determine the effects of longitudinal loading (artificial gravity), on skeletal muscle protein kinetics in fifteen healthy young males after 21 days of 6° head-down tilt bed rest [treatment group (TRT); n = 8; 31 ± 1 and control group (CON); n = 7; 28 ± 1y] (mean ± SEM). On day 1 and 21 of bed rest, postabsorptive venous blood samples and muscle biopsies (vastus lateralis and soleus) were obtained during a one hour pulse bolus infusion protocol [0 min., L- [ring-¹³C₆] phenylalanine (35 µmol/kg) and 30 min., L- [ring-¹⁵N] phenylalanine (35 µmol/kg)]. Outcome measures included mixed muscle fractional synthesis (FSR) and breakdown (FBR) rate. The TRT group experienced 1 h of longitudinal loading (2.5 G at the feet) via a short radius centrifuge during each day of bed rest. Mixed muscle FSR in the CON group was reduced by 48.5% [0.081 ± 0.000 (d 1) vs. 0.042 ± 0.000 %.h^-1 (d 21), p = 0.001] in vastus lateralis after 21 days of bed rest, whereas the TRT group maintained their rate of protein synthesis. A similar, but non-significant change in FSR was noted for the soleus muscle (TRT, -7% and CON, -22%). No changes in muscle protein breakdown were observed. In conclusion, 1 h daily exposure to artificial gravity maintained the rate of protein synthesis of the vastus lateralis and may represent an effective adjunct countermeasure to combat the loss of muscle mass and functional during extended spaceflight.