Diminished muscular activity is associated with alterations of protein metabolism. The aim of this study was to evaluate the effect of short-term muscle inactivity on regulation of whole-body protein deposition during amino acid infusion to simulate an experimental postprandial state. We studied nine healthy young volunteers at the end of 14 day periods of strict bed rest and of controlled ambulation using a cross-over design. Subjects received a weight-maintaining diet containing 1 g protein kg(-1) day(-1). l[1-(13)C]leucine was used as a marker of whole-body protein kinetics in the postabsorptive state and during a 3 h infusion of an amino acid mixture (0.13 g amino acid (kg lean body mass)(-1) h(-1)). In the postabsorptive state, bed rest decreased (P < 0.05) the rate of leucine disposal (R(d)) to protein synthesis and tended to decrease leucine rate of appearance (R(a)) from proteolysis, whereas the rate of leucine oxidation did not change significantly. Amino acid infusion increased leucine R(d) to protein synthesis and oxidation and decreased leucine R(a) from proteolysis in both the bed rest and ambulatory conditions. Changes from basal in leucine R(d) to protein synthesis were lower (P < 0.05) during bed rest than those in the ambulatory period, whereas changes in leucine R(a) from proteolysis and oxidation were not significantly different. During amino acid infusion, net leucine deposition into body protein was 8 +/- 3% lower during bed rest than during the ambulatory phase. In conclusion, short-term bed rest leads to reduced stimulation of whole-body protein synthesis by amino acid administration. Results of this study were, in part, presented at the meeting, Experimental Biology, 2004, Washington DC.