an experiment was carried out to evaluate the performance and nutrients digestibility by broiler chickens in the period of 1 to 21 days of age. the experimental design was completely randomized, where t1 was the control treatment constituted by 21.1% of crude protein (cp), 3000 kcal of apparent metabolizable energy (amen)/kg, 0.46% of available phosphorus (ap) and 1.0% of calcium (ca). t2 was the diet with 17.0% of cp and 3000 kcal of amen/kg, t3 the diet with 19.0% of cp and 2900 kcal of amen/kg, t4 the diet with 19.0% of cp and 3000 kcal of amen/kg, and t5 the diet with 19.0% of cp and 3100 kcal of amen/kg. except for the control diet, the diets were formulated with 0.34% of ap, 0.80% of ca and supplemented with phytase. the weight gains of the chickens were similar (p>0.05), independently of the experimental diets. however, the worst feed conversion was observed for the birds fed with 17.0% of cp. the birds which were fed diets with 19.0% of cp and 3000 or 3100 kcal of amen/kg showed performance similar to those fed with the control diet. the excretion of n and p reduced, on average, 16.5% and 22.0%, respectively, and there was increase in the p retention for the birds that were fed the experimental rations. the retention of ca and n was smaller and similar, respectively, for the birds that consumed experimental rations. from these results, it is concluded that the diet with 19.0% of cp and 3000 kcal of amen/kg, supplemented with phytase, does not affect the 1-to-21-day-old broiler chickens and enables to decrease the environmental impact of the nutrients.