This experiment was conducted to evaluate the effects of dietary crude protein [CP; normal and low (90% of the normal CP level)] and guanidinoacetic acid (GAA; 0, 0.6, and 1.2 g/kg of diet) on growth performance, leukocyte profile, stress indicators, antioxidant enzymes activity, and intestinal morphology in broiler chickens under cyclic heat stress (HS). A total of 504 one-d-old male broiler chickens (Ross 308) were randomly assigned to seven treatment groups (6 cages/treatment with 12 birds/cage). The positive control (PC) broiler chickens were housed in a thermoneutral chamber and fed with a basal diet (normal diet without GAA supplementation). The other six groups were kept in a cyclic HS chamber (34 °C) for 8 h (10:00-18:00). All experimental diets were fortified with synthetic feed-grade methionine, lysine, threonine, and valine at levels sufficient to meet dietary requirements. Cyclic HS decreased (P < 0.001 or 0.05) overall average daily gain (ADG), European performance index, and serum T4 level, while increased overall feed conversion ratio (FCR) and heterophil to lymphocyte ratio compared to the PC group. There was a CP × GAA interaction for overall ADG (P = 0.025), indicating that the effect of GAA on this response was more marked at low dietary CP levels. Heat-stressed broilers that had received the low-CP diet, amino acid-fortified diets could support growth performance and physiological traits equivalent to those that had received the normal-CP diets. Dietary inclusion of GAA caused significant linear increases in villus height (P = 0.007), villus width (P = 0.021), villus height to crypt depth ratio (P = 0.002) and villus surface area (P = 0.001) in the duodenum, as well as villus surface area (P = 0.043) in the jejunum. An increase in dietary GAA levels was also accompanied by a linear increase (P = 0.041) in the serum T3 level. In conclusion, the positive effects of GAA supplementation were clearly evident for broilers reared under the cyclic HS cond
