<p>Published literature indicates that L-arginine (L-Arg), guanidinoacetic acid (GAA) and L-citrulline (L-Cit) can all provide arginine activity in broiler diets (Su and Austic, 1999; Dilger et al., 2013; DeGroot, 2018) and all are commercially available. GAA has been reported to have 77% arginine equivalence for feed conversion (Ringel et al., 2013). This study was conducted to determine the effect of arginine deficiency on performance in reduced protein diets and the efficacy of adding it back as either L-Arg, GAA or L-Cit. Day old male Ross 308 birds (n = 768) were assigned to 8 dietary treatments using a completely randomized design: normal protein (NP, CP levels of 24.9, 23.7 and 21.4% for starter, grower and finisher, respectively), reduced protein deficient in Arg (RP, CP levels 5% lower than those of NP diets for all feeding phases) and RP with 2 levels of L-Arg (238 and 476 g/kg), GAA (309 and 618 g/kg) or L-Cit (238 and 476 g/kg).</p>