Recent studies have shown that the relative feed intake (FI) and bodyweight gain (BWG) responses of chronically heat-stressed broilers to equimolar additions of DL-methionine (DLM) or 2-hydroxy-4-(methylthio) butanoic acid (HMB; Alimet) can be altered by changing the arginine:lysine (arg:lys) ratio of the diet. However, no information is available concerning responses of broilers during acute heat stress. Broilers were exposed to either 22°C or 30°C during 3 consecutive 5-day experimental periods between 28 and 43 days of age. Responses to equimolar additions of the 2 sources of supplemental methionine activity were similar during exposure to 22°C at 28-33 days and 38-43 days of age. However, between 33 and 38 days of age, when the temperature was 30°C, increasing the dietary arg:lys ratio from 1.04 to 1.35 significantly improved the FI and BWG of broilers fed HMB, but not DLM. At an arg:lys ratio of 1.35, the BWG of broilers fed HMB was significantly greater than that of broilers fed DLM. Therefore, under acute heat stress conditions the choice of a supplemental source of methionine activity should be dependent upon the dietary arg:lys ratio. A R 0 2 1 1 7 R e s p o n s e o f b r o i l e r s t o a r g : l y s r a t i o J . C h e n e t a l .Additional keywords: feed intake, bodyweight gain, DL-methionine, 2-hydroxy-4-(methylthio) butanoic acid, Alimet, acute heat stress. Australian Journal of Agricultural ResearchJ. Chen et al.