We measured the 13C enrichments of expired CO2 and deuterium enrichments of plasma free lysine and VLDL-apolipoprotein B-100 in five nulliparous women who received an oral bolus dose of [1-13C]leucine and a primed, constant infusion of [2H4]lysine on d 2 and 6 while consuming protein diets of 1.5, 1.0 and 0.4 g.kg-1 x d-1. Peak enrichments and cumulative percent recoveries of 13C in expired CO2 increased during the high, and decreased during the low protein periods within 24 h of altered intakes; these changes averaged 89% of that on d 6 of the high and low protein diets. The early changes in leucine oxidation showed significant relationships with urinary nitrogen excretion on d 6 of the dietary periods. The ratio of the isotopic enrichment of lysine in VLDL-apolipoprotein B-100 to that in plasma was unaltered by the level of protein intake. Thus, amino acid oxidation adapts rapidly to altered protein intakes in adult women and is useful to determine protein needs during rapidly changing physiologic conditions. However, 6-d periods of protein intake over the range of 1.5 to 0.4 g.kg-1 x d-1 do not affect the proportional use of dietary amino acids for hepatic secretory protein synthesis.