The nitrogen (N) retention response to the abomasal infusion of amino acids in South African Mutton Merino ram lambs fed a standard finishing diet was investigated. The basal diet consisted of (g/kg air dry weight): 300 lucerne, 80 wheat straw, 561.5 maize meal, 35 molasses meal, 10 urea, 5 salt, 7.5 ammonium chloride, 1.0 vitamin/mineral premix and 0.29 taurotec®. Twenty-eight lambs (ca. 38 kg live weight) were randomly allocated to four treatments consisting of different abomasal infusion rates of an amino acid mixture, formulated to contain essential amino acids in the same ratio as that of the whole empty body. The mixture consisted of (g/kg): 119.7 arginine, 154.9 histidine, 22.1 isoleucine, 106.5 leucine, 91.5 lysine, 146.9 methionine, 105.6 phenylalanine, 133.8 threonine and 118.7 valine. The mixture was infused at levels of (g/d): 0.00, 38.27, 51.02 or 63.78. Glycine was used to balance the various mixtures on an iso-nitrogenous basis. Infusion of 51.02 g/d of the essential amino acid mixture resulted in a significant increase in N retention, expressed as a percentage of N intake, compared to the control group which received the test diet and glycine (phase 1). The omission of histidine, methionine or threonine from the infusate (phase 2) had no significant decrease in N retention or plasma amino acid concentrations compared to the infusion of the mixture of nine essential amino acids.