The metabolic mechanisms of accommodation to a low-protein diet in elderly women were studied. Diets containing 1.47 (low) or 2.94 (adequate) g protein.kg body cell mass-1.d-1 (0.45 or 0.92 g.kg body weight-1.d-1, respectively) were fed for 10 wk. Fed state leucine kinetics, fasted and fed metabolic rate, and acute-phase responses to a bout of exercise were measured. Leucine uptake into protein, breakdown, and acute-phase responses were not significantly different within or between diet groups. In the low-protein group, leucine flux and oxidation were lower after 3 wk and oxidation continued to decrease throughout the 10-wk study. When expressed per body cell mass, metabolic rate increased over time in the low-protein group only. Leucine oxidation was a more sensitive index of the adequacy of protein intake than synthesis, flux, metabolic rate, or acute-phase response.