Nitrogen retention and lysine oxidation were measured in growing pigs given diets which supplied 0, 0.2 or 0.8 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 L-[6-3H]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)0.75 per d) for the three diets respectively were for N retention, 0.00, 0.32 and 1.22, and for lysine oxidation 0.051, 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 0.85; from the oxidation results (assuming lysine absorbed but not retained is oxidized) the estimate was 0.95.