2013. Effects of different heat stress periods on various blood and meat quality parameters in young Arbor Acer broiler chickens. Can. J. Anim. Sci. 93: 453Á460. Heat stress can influence muscle metabolism and meat quality in animals reared for food production. From a commercial perspective, understanding the mechanism of this effect is clearly necessary. The aim of this study was to investigate the effects of different heat stress periods on serum metabolites and chicken meat quality (n0120). Plasma indicators creatine kinase (CK), glutamic-pyruvic transaminase (GPT), glutamic-oxaloacetic transaminase (GOT), insulin and glucagon and meat quality (pH, expressible moisture, cooking losses, shear force values) were evaluated. Compared with controls, the concentrations of CK and GPT increased (P B0.01) after 2 and 3 h of heat stress, respectively, whereas plasma insulin and glucagon decreased after 1 and 5 h of heat stress, respectively. The immediate pH (pHi) and ultimate pH (pHu) of the pectoralis muscles decreased (PB0.01) after 1 and 2 h of exposure to heat stress, respectively. Cooking loss, expressible moisture and shear force value increased (P B0.01) after 3, 2, and 1 h of heat stress, respectively. These data indicate that elevated plasma concentrations of CK and GPT can be used as parameters for assessing the stress level to which broilers are exposed before slaughter. The preslaughter exposure of broiler chickens to heat stress can alter muscle metabolism and membrane integrity, leading to undesirable meat characteristics. Thus, a clear understanding of the mechanisms underlying these processes will contribute to the determination of prevention strategies and the avoidance of the associated economic losses.