We examined the effects of recombinant human insulin-like growth factor I (IGF-I) and insulin on the plasma amino acid (AA) profile and leucine kinetics in eight normal subjects. IGF-I was infused at 52 pmol.kg-1.min-1, in combination with prime-continuous [1-14C] leucine infusion, to obtain steady-state plasma concentrations of total (54 +/- 3 nmol/l) and free (7.3 +/- 1 nmol/l) IGF-I (study 1). In response to IGF-I, plasma AA levels declined by 37 +/- 3% (1975 +/- 198 to 1368 +/- 120 mumol/l) and total branched chain amino acids (BCAA) declined by 34 +/- 3% (390 +/- 21 to 256 +/- 13 mumol/l). This hypoaminoacidaemic effect was associated with a decline in endogenous leucine flux of 17 +/- 2% (1.88 +/- 0.05 to 1.57 +/- 0.04 mumol.kg-1.min-1) and leucine oxidation of 17 +/- 1% (0.31 +/- 0.02 vs 0.26 +/- 0.02 mumol.kg-1.min-1) (both p < 0.01 vs basal). The same subjects underwent a second study (study 2) in which insulin was infused at 6.22 pmol.kg-1.min-1 to obtain a steady-state plasma insulin concentration of 530 +/- 25 pmol/l while maintaining euglycaemia. The infusion rate was designed to match the declines in plasma BCAA levels and leucine turnover observed during IGF-I infusion. The rates of glucose infusion necessary to maintain euglycaemia during IGF-I and insulin infusion were 4.9 +/- 1.0 and 7.8 +/- 0.6 mg.kg-1.min-1, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)