A 42-d trial was conducted with 252 broiler chickens (d 1 of age, 38.8 +/- 1.3 g of BW) to determine the effects of Forsythia suspensa extract on growth performance, nutrient digestibility, and antioxidant activities under high ambient temperature (32 +/- 1 degrees C). The feeding program consisted of a starter diet from d 1 to 21 of age and a finisher diet from d 22 to 42 of age. Dietary treatments included a negative control group (NC) fed a cornsoybean meal based diet without vitamin C or Forsythia suspensa extract, a positive control group fed a diet with 200 mg of vitamin C/kg, and a test group (FS) fed with 100 mg of Forsythia suspensa extract/kg. There were 14 cages per treatment and 6 birds per cage during the study. Birds had free access to diets and water during the entire period. Body weight and feed intake were measured at d 21 and 42. Blood and tissue samples were collected at d 21 and 42 for assay of antioxidant indices. Growth performance did not differ among treatment groups during the starter period. In the finisher phase, birds in FS had greater (P < 0.05) average daily gain, feed conversion, and apparent digestibility of energy, CP, calcium, and phosphorus than birds in NC. Furthermore, birds in FS had greater (P < 0.05) total antioxidant capacity and superoxide dismutase activity and lower (P < 0.05) malondialdehyde activity in the serum than birds in NC. The FS birds had greater (P < 0.05) muscle superoxide dismutase activity and lower (P < 0.05) malondialdehyde than NC birds. During the entire period, hepatic superoxide dismutase activity of FS birds was greater (P < 0.05) than that of NC birds. Dietary supplementation with Forsythia suspensa extract can enhance nutrient digestibility and growth performance possibly by reducing oxidative stress of broiler chickens under high ambient temperatures.