Two 14-d experiments were conducted using 540 and 450 pigs to determine the Lys requirement for 7 to 16 kg pigs when feeding a variety of synthetic amino acid concentrations, intact protein concentrations, or diets with low protein concentrations. Pigs were weighed 7d after the completion of the experiments (d 21) to determine the carryover effects of treatment diets. Performance data revealed no (P > 0.17) protein source × Lys level interactions. In Exp. 1, ADG increased quadratically (P = 0.04) with increasing Lys level and Lys:CP, while G:F increased in a linear (P < 0.0001) manner. Break line analyses of all treatments utilizing analyzed SID Lys concentrations revealed optimum (P = 0.001) ADG was obtained at 1.26% Lys, while optimum G:F (P = 0.002) was obtained at 1.34% Lys. The source of NEAA affected (P > 0.08) neither ADG nor G:F. In Exp.2, both ADG and G:F increased linearly (P < 0.0001) with increasing Lys level. Optimum (P = 0.001) ADG and G:F were both obtained at 1.29% Lys. The source of Lys did not affect (P = 0.57) ADG, but tended to affect (P = 0.07) G:F. Overall, these data suggest that ADG was improved with increasing Lys levels up to 1.29%. While the source of Lys did not affect ADG, supplying Lys from L-Lys•HCl compared to SBM tended to improve feed efficiency.