The effect of Arg:Lys, Met source, and time of exposure to heat stress on growth and body protein accretion was tested in acutely heat-stressed (AHS) or chronically heat-stressed (CHS) broilers. Ross 308 1-day-old chickens were raised under normal brooding conditions from 1 to 25 d of age and were then suddenly moved to 32 degrees C (AHS), whereas another group was kept at constant high temperatures throughout the grow-out period (32 degrees C; CHS). From 26 to 33 d of age, both groups were therefore at 32.8 +/- 1.0 degree C. Two rooms were used per environmental treatment. A basal diet deficient in TSAA was supplemented with L-Arg monohydrochloride to achieve Arg:Lys ratios of 0.95 and 1.40. Diets were supplemented with either L-Met, 2-hydroxy-4-(methylthio) butanoic acid (HMB), or DL-Met (DLM) to a level of TSAA 5% lower than requirements. Each Arg:Lys and Met source combination was diluted with a N-free diet to achieve graded levels of CP (0.0, 3.5, 7.0, 10.5, 14.0, and 17.5% CP) and fed to 18 replicates of 3 chickens (3 replicates per level). Treatment effects were obtained by the slope-ratio technique using average daily BW gain and body CP deposition as dependent variables and protein intake as the independent variable. Protein utilization remained unaffected by Met source when fed at high Arg:Lys for birds under AHS and CHS (P > 0.05). However, lower protein utilization was observed in birds fed L-Met in low Arg:Lys compared with those fed DLM (P < 0.05). Birds fed HMB at low Arg:Lys utilized dietary protein better than those fed L-Met only under CHS conditions (P < 0.05). Protein utilization for birds fed HMB was similar to that of birds fed DLM in all instances. It was concluded that Arg:Lys, Met source, and time of exposure to heat stress affected protein utilization in hyperthermic birds.