Normal performance can be achieved by feeding low crude protein (LCP) finisher diets when supplemented with adequate essential amino acids. The question remains as to the adequacy of LCP diets under high temperature stress. The performance of meat chickens was determined when fed an LCP finisher diet while exposed to an acute (AT) or chronic (CT) thermal challenge or thermo-neutral conditions (TN).Day-old male Ross 308 chicks (N =360) were reared in floor pens until d 28 and then transferred to metabolism cages (5 birds/cage) and housed in environmentally controlled metabolism rooms, maintained at 20°C until d 38 of age. In room 1, birds were exposed to CT, 28 °C for 5 h/d on d 38-42 (22 cages). Room 2 was maintained at 20°C on d 38-39 and then increased rapidly to 30-32°C for 3 h/d (AT) on d 40-42 (22 cages). The birds in the remaining 22 cages of room 2, were removed 1 h before the ATand held in a third room at TN and returned to room 2 when the temperature had returned to 20°C. For each temperature treatment equal number of cages were fed either a control finisher (20.3 % protein) or LCP diet (17% protein) on d 28-42 of age.Individual bodyweights and pen feed intake were determined on d 28 and d 38 (TN and CT) and d 40 (TN and AT) and all on d 42. Total excreta were collected d 40-42 to determine AME, AMEn and % protein retention. On d 43 breast weight and the % breast muscle yield were determined. Bird behaviours were recorded over d 38-42 of age.The AT resulted in a 32.3 % reduced LWG, had no effect on feed intake but FCR was 22.3 % poorer. The CT had no adverse effects on bird performance. The diet CP had no effects on performance under either challenge. The thermal challenges or diet CP had no effects on breast muscle yields, AME, AMEn or % protein retention. Depending on the thermal condition birds spent 73-90 % of their time crouching.The absence of any adverse effects of the CT suggest it was of moderate severity and birds coped. However, based on the reduced performance, mortality and removals, and high % of birds panting, the AT caused ‘severe’ to ‘life-threating’ conditions. The reduction in CP had no consequence under these thermal conditions.