Leucine (LEU) kinetics were assessed using a primed-continuous infusion of L-[1-14C]LEU in normal overnight-fasted male volunteers during a basal period and an experimental period where insulin (INS) was infused at either 0.6, 1.2, 2.5, 5.0, 10, or 20 mU · kg-1 · min-1 with euglycemia maintained. Two protocols were used: 1) subjects were allowed to develop hypoaminoacidemia or 2) plasma essential amino acids (AA) were maintained near basal levels by frequently monitoring plasma LEU in conjunction with variable infusions of an AA solution (LEU infused = 0.41, 0.72, 0.93, 1.03, 1.31, and 1.35 μmol · kg-1 · min-1 at escalating INS doses, respectively). Basal rates of LEU appearance (R(a)), non-oxidative disappearance (NOR(d)) and oxidative disappearance (OXR(d)) were similar in both protocols (means = 1.74, 1.40, and 0.36 μmol · kg-1 · min-1, respectively). INS infusions without AA resulted in a progressive decrement in LEU R(a) (14 to 45%), NOR(d) (16-41%), and OXR(d) (3-56%) compared with basal values. The infusion of AA resulted in an additional reduction in endogenous R(a) (P < 0.01; ~ 100% suppression achieved at plasma INS > 1,000 μU/ml) and a blunting of NOR(d) reduction (P < 0.05) at each dose of INS. Observed differences in INS's suppression of LEU R(a) between the two protocols suggests the existence of a component of whole body proteolysis that is highly dependent on circulating plasma AA. Therefore, hypoaminoacidemia associated with INS treatment would appear to blunt the responsiveness of INS's suppression of protein breakdown and in the presence of near basal plasma AA, proteolytic suppression by INS is enhanced.