Three experiments were conducted to study the effect of dietary protein level on the retention of nutrients, growth performance, litter composition and NH3 emission in broiler chickens kept under laboratory conditions (housed in cages, Exp.1) or in commercial conditions (in pens, Exp.2; or whole houses of a farm, Exp.3). All the trials were performed according to a factorial experimental design, involving a 4-stage feeding programme and two levels of dietary crude protein (CP) for each period: control vs. low crude protein (CP reduced by 1.5%). In Exp.1, the coefficients of total tract apparent retention of dry matter and CP were higher in the birds fed the low CP diet (p