The purpose of this review is to consider the distinct possibility that dietary non-bound and protein-bound amino acids are not bioequivalent in broiler chickens. Usually, with conservative inclusions of a limited number of non-bound (synthetic, crystalline, feed-grade) amino acids in standard broiler diets, bioequivalency would not be an issue. However, reduced-crude protein (CP) broiler diets demand substantial inclusions of an extended range of non-bound amino acids to meet amino acid requirements. A standard diet may contain 5.0 g/kg non-bound amino acids, but a reduced-CP diet may contain up to 50 g/kg and this relative abundance skews the balance of non-bound to protein-bound amino acids and substantial proportions of certain amino acids are present in diets as non-bound entities. Importantly, tangible reductions in dietary CP, for example from 210 to 160 g/kg, usually both compromise broiler growth performance and increase fat deposition. Compromised growth performance is more evident in wheat- than maize-based diets but, paradoxically, fat deposition is more apparent in maize-based diets. The inability of birds to accommodate tangible dietary CP reductions appears to stem partially from the lack of bioequivalency between non-bound and protein-bound amino acids because of the differentials in intestinal uptake rates. Also, reduced-CP broiler diets generate perturbations in apparent amino acid digestibility coefficients which compound the fact that intestinal uptakes of non-bound acid acids are more rapid, and occur more anteriorly in the small intestine, than protein-bound amino acids. The likelihood is that greater proportions of non-bound amino acids transit the enterocytes of the gut mucosa without entering anabolic and/or catabolic pathways to gain entry to the portal circulation. This culminates in post-enteral amino acid imbalances and postprandial oxidation of surplus amino acids which involves deamination of amino acids and elevations in plasma ammonia (NH{}_3) concentrations, but NH{}_3 is inherently toxic and demands detoxification. Excessive deamination coupled with inadequate detoxification could result in ‘ammonia overload’ which would be expected to compromise growth performance. Thus, the hypothesis is that non-bound and protein-bound amino acids are not bioequivalent; moreover, it may be argued that this distinction is being overlooked and is thwarting the development and acceptance of reduced-CP broiler diets.