We determined the effect of a 4-h leucine infusion, leading to 15-fold elevated plasma leucine concentrations, on skeletal muscle and whole-body protein synthesis in suckling lambs during the postprandial period. The [3H]phenylalanine large dose method was validated and used to quantify the fractional rates of protein synthesis (Ks in %/d) at the end of the leucine infusion. In the first experiment leucine infusion lowered plasma amino acid concentrations but did not change the Ks, the capacity for protein synthesis (Cs, mg RNA/g protein) or the efficiency of translation [g protein synthesized/(d-g RNA)] in any muscles studied or the whole body. In the second experiment the leucine-induced decreases in plasma amino acid concentrations were compensated by the simultaneous infusion of substantial amounts of amino acids. Again leucine excess did not significantly change Ks, Cs and efficiency of protein synthesis. These results indicated that leucine excess in suckling lambs during the postprandial period lowered aminoacidemia without any change of the protein synthesis rates in skeletal muscles or the whole body.