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In two experiments, the effects of arginine:lysine, NaHCO3 and methionine source on performance and amino acid digestion of heat-stressed broilers were studied. In exp. 1, corn/soy diets with argine:lysine (Arg:Lys) of 1.10, 1.25 and 1.40 were fed from 21 to 42 d. Diets were supplemented with 2-hydroxy-4-(methylthio)-butanoic acid (HMB) or DL-Methionine (DLM) with NaHCO3 at 0 or 1.04%. Ten replicates per treatment of four birds per cage were maintained at 31.7 ± 0.9°C. From 21 to 28 d weight gain (BWG), feed consumption and feed-to-gain (FCR) were unaffected by diet treatment. Methionine source did not influence bird performance (P > 0.05). However, from 35 to 42 d, when diets were supplemented with NaHCO3, BWG and FCR of birds fed DLM were compromised when offered diets with Arg:Lys at 1.40. In diets devoid of NaHCO3, increasing Arg:Lys to 1.25 impaired BWG and FCR for broilers fed DLM, although effects disappeared when more Arg was used. Performance of birds on HMB was mostly unaffected by Arg:Lys or NaHCO3 except when fed diets void of NaHCO3 with Arg:Lys of 1.40, when FCR was impaired. In exp. 2, broilers were maintained at thermoneutrality (TN) reaching 20°C at 33 d. Other birds were at 31°C throughout, representing chronic heat stress (CHS), while a third group changed suddenly from 20 to 31°C at 25 d representing acute heat stress (AHS). Birds within each environment were housed in one of two rooms with eight replicates of four birds per treatment. At 25 d, birds were fed Arg:Lys of 0.95 or 1.40 with DLM supplemented at requirement. Birds fed Arg:Lys of 1.40 were also offered diets with synthetic L-methionine (L-Met). Adding L-Met to high Arg diets during CHS significantly depressed ileal digestibility of all amino acids (P < 0.05). It is concluded that chronic heat stress can affect the digestibility of amino acids, and that this effect may impact performance. Key words: Broilers, methionine, arginine, heat stress, digestibility, sodium bicarbonate