Guanidinoacetic acid (GAA) is synthesized from Gly and Arg and the immediate precursor of creatine. The creatine (Cr) and creatine phosphate (PCr) system plays a critical role inenergy metabolism. It ishypothesized that dietary supplementation with GAA will be beneficial to heat stressed birds finishing broilers. A total of 720 one-day-old male Ross 308 broilers were allocated to 3 dietary treatnments with 12 replicates (20 birds each). Treatments were either 0, 0.6 or 1.2 g/kg GAA added to a corn/ SBM diet and fed for 39d. A chronic cyclic heat stress model (340C/540-60% rh for h daily) was applied in the finisher phase (d25-39). One bird per pen was sampled on d 26 (acute heat stress) and d39 (chronic heat stress) to determine levels of GAA, Cr and Arg in blood, and Cr, PCr and ATP in muscle. GAA at 1.2 g/kg decreased F:G compared to control diet in the grower phase (d10-25) (1.32 vs 1.35; P<0.05). In the heat stressed finisher period, 0.6 and 1.2 g/kg GAA reduced feed intake by 1.1 and 3.3% respectively and improved F:G (1.76, 1.66 and 1.67 for control, 0.6 and 1.2 g/kg GAA respectively, P<0.05). Examination of breast meat samples revealed at both sampling days significantly (P<0.05) increases of PCr, Cr, and PCr:ATP ratio with increasing dietary GAA. GAA and Cr in blood were increased with increasing dietary GAA (P<0.05) at d26 and d39. Blood Arg increased with increasing dietary GAA (+18.2 and +19.9% for 0.6 g/kg GAA on d26 and d39, respectively,and +30.8 and +33.6% for 1.2 g/kg GAA, P<0.05) suggesting enhanced availability of Arg for other metabolic purposes than de-novo GAA foremation.

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