Background: Bed rest is associated with a loss of protein from the weight bearing muscle. Objectives: To determine whether increasing the amount of the branched chain amino acids (BCAAs) in the diet during bed rest would improve the anabolic response after bed rest. Methods: The study was divided into three phases, a one day ambulatory period, fourteen days of bed rest period and a four day recovery period. Before starting the bed rest phase, the subjects (n=12) were randomized into two groups. Dietary intake (1.3 x Resting energy expenditure) was supplemented with either 30 mMol.d^-1 each of glycine (2.16 g d^-1), serine (3.15 g d^-1) and alanine (2.58 g d^-1) and diet 2 (BCAA supplemented) with 30 mMol.d^-1 each of the three branched chain amino acids leucine (3.93 g d^-1), isoleucine (3.93 g d^-1) and valine (3.51 g d^-1. Protein and glucose kinetics were measured during each phase. Whole body protein synthesis (WBPSR) was determined with U-¹⁵N labeled algal amino acids, muscle (M-FSR) and plasma protein synthesis (fibrinogen, ceruloplasmin, complement C-3, transferrin and VLDL Apoprotein B-100, PP-FSR) with ²H₅ L-phenylalanine. Total glucose production (Ra_T) and gluconeogenesis from alanine (Ra_Gng) were determined by with a combination of U-¹³C₃ L-alanine and 6,6 ²H₂ glucose . Daily N balance and urinary 3-methyylhistidine excretion were determined for the bed rest and recovery phases. Results: N retention was greater with BCAA fed group during the second week of bed rest (56 ± 6 vs 26 ± 12 mg N. kg^-1.d^-1, p<0.05). WBPSR was reduced by ~20% during bed rest (NEAA, 3.39 ± 0.10 vs 2.86 ± 0.18 g prot. kg^-1.d^-1, p<0.05; BCAA 3.96 ± 0.33 vs 2.89 ± 0.27 g prot. kg^-1.d^-1, p<0.05). There was no effect of BCAA supplementation on either M-FSR (0.34 ± 0.15 vs 0.34 ± 0.08 %. d^-1), or PP-FSR or the rate of 3-MeH excretion (3.55 ± 0.10 vs 3.26 ± 0.10 µmol.kg^-1.d^-1) during or after bed rest. Muscle tissue free amino acid concentrations were increased during bed rest with BCAA (0.214 ± 0.066 vs 0.088 ± 0.12 nmol. mg protein^-1, p<0.05). Ra_T and Ra_AGng were unchanged with bed rest but were reduced (p<0.05) with the BCAA group in the recovery phase (Ra_T 22.2 ± 1.0 vs 18.6 ± 0.9 mmol. kg^-1.hr^-1 and Ra_T (0.72 ± 0.10 vs 0.34 ± 0.12 mmol. kg^-1.hr^-1). Conclusions: Accretion of amino acids in the tissue free amino acid pools is a significant factor in the improved N balance found with BCAA supplementation during bed rest. The amount accreted is not enough to impact protein kinetics in the recovery phase, but does improve N retention by providing additional essential amino acids in the early recovery phase.