Two studies were conducted to investigate effects of supplementing 100 or 130% of the required digestible methionine to corn-soybean meal-based grower and finisher diets (1) as DL-methionine (DL-MET) for broilers housed in a higher density and (2) as either DL-MET or 2-hydroxy-4-(methylthio)butanoate (HMTBA) for broilers exposed to a high ambient temperature. In the first study, the high density impaired (P < 0.05) growth performance in both phases, decreased (P < 0.05) liver and adipose tissue fatty acid concentrations, and increased (P < 0.05) glutathione (GSH) in all assayed tissues except for the liver of the growers. The 130% DL-MET supplementation decreased (P < 0.05) feed intake of the finishers as well as the finisher breast and thigh malondialdehyde (MDA) levels. DL-MET elevated (P < 0.05) GSH in the grower thigh and fatty acid concentrations in the finisher liver. The hepatic expressions of heat shock protein 90 (HSP90) was decreased (P < 0.05) by the extra methionine supplementation. In the second study, the 130% methionine supplementations of both forms enhanced (P < 0.05) hepatic GSH concentrations of the growers and ferric reducing ability of plasma (FRAP) of the finishers. The DL-MET-fed growers had greater (P < 0.05%) muscle GSH and hepatic unsaturated fatty acid concentrations than those fed HMTBA. Expression of inflammation-related genes in the liver of finishers was affected (P < 0.05) by interaction effects of the methionine form and concentration. In summary, extra methionine supplementation showed moderate beneficial effects on tissue antioxidant status and growth performance of broilers under environmental stresses.