Nitrogen retention and lysine oxidation were measured in growing pigs given diets which supplied 0,0-2 or 08 of the lysine requirement, with other amino acids in relative excess. Eight groups of three female littermate pigs were used: one of each group was given each of the three diets. In half the pigs (four groups) N retention was measured at body weights (W) of approximately 25,35 and 45 kg. The other four littermate groups of three pigs were given the same three diets; when they reached 35 kg W they were given a continuous (6 h) primed infusion of ~-[6-~H]lysine. Lysine oxidation was estimated from the production of tritiated water. Rates of both N retention and lysine oxidation increased significantly with lysine intake; mean values (g/kg W@75 per d) for the three diets respectively were for N retention, 0.00, 0.32 and 1.22, and for lysine oxidation 0051, 0.058 and 0.078. From the N balance results (assuming a constant lysine concentration in body protein) the efficiency of utilization of absorbed lysine was estimated to be 085; from the oxidation results (assuming lysine absorbed but not retained is oxidized) the estimate was 095.