the experiment aimed to test different ratios digestible lysine:crude protein (cp) in diets for broiler until 21 days. one thousand and eight hundred 1-d birds of cobb strain were allotted to a completely randomized design with a hierarchical rank model with three levels of cp and four ratios digestible lysine:cp was used. in the performance trial, the diets were fed to 30 birds in each of the five replicates. at 21 days of age, feed intake, weight gain and feed conversion were evaluated. a linear increase on weight gain was observed in the 18.5 and 21.5% of cp levels. feed conversion improved linearly according to the ratios of lysine within the three levels of cp studied. carcass yield increased linearly with increasing rations at the level of 15.5% of cp, it not being significant at the other levels of cp studied. in the metabolism trial, the ratios digestible lysine:cp showed linear effect on eman (21.5% of cp) and nitrogen excretion (18.5% of cp), quadratic effect on eman (15.5 and 18.5% of cp) and n retention coefficient (15.5% of cp). based on the results of weight gain, the ideal digestible lysine:cp ratio in the initial phase was of 6.8% (18.5% of cp) and 5.9% (21.5% of cp), or 1.26 and 1.27% of digestible lysine. the dietary cp contents fed to broilers in the initial phase can be reduced to 18.5%, with no effect on bird performance, but by decreasing nitrogen excretion to 24%, since the diets are supplemented with synthetic aminoacids.