In the present study the plasma amino acid response of six lean subjects to a protein meal and a commercial amino acid supplement were compared. The amino acid supplement studied was formulated and marketed to be taken after exercise and at other times with the aim of increasing protein synthesis and/or decreasing protein degradation and to lower the ratio of tryptophan to the other large neutral amino acids (LNAA); tyrosine, valine, leucine, isoleucine, phenylalanine and methionine (trp/LNAA), to reduce fatigue. The amino acid supplement administered at the dose recommended by the manufacturer (4 g) was able to bring about a rapid but short-lived (15-30 min) increase in plasma amino acid concentrations and to produce a similarly brief decrease in the trp/LNAA and tyr/LNAA ratios and therefore achieved these aims with respect to amino acid levels even if only briefly. The changes in trp/LNAA and tyr/LNAA ratios after the supplement were of the same order as those produced after the much larger (50 g) protein meal but of shorter duration. However the relatively small insulin response after the amino acid supplement points to a lower level of amino acid uptake by muscle and other tissues for protein synthesis compared to that produced by the beef meal.