Technical Abstract: Heat stress in feedlot cattle causes reduced performance, and in the most severe cases, death of the animals, thus causing the loss of millions of dollars in revenue to the cattle industry. A study was designed to evaluate the dynamics of thermoregulation and feeding activities when feeder cattle were exposed to simulated heat waves in comparison with repeated sinusoidal hot and thermoneutral environments. Nine beef steers were randomly assigned to an individual pen in one of three environmental chambers. Each chamber was subjected to each of three temperature regimes (Heatwave simulation from Rockport, MO 1995, Heatwave simulation from Columbia, MO 1999, and Controlled heat stress treatment of 32±7C) for a period of 18 days, according to a Latin square treatment design with a 10-day thermoneutral period (18±7C) separating treatment periods. Respiration rate, core body temperature, and feed intake were measured on each animal for the duration of the experiment. Differences were found in all treatments for all parameters; however it was very apparent that severity of the stress was dependent on the response variable of interest. The response of respiration rate to temperature was found to be similar in all animals and have little acclimation effect. Therefore overall recommendation is that respiration rate was found to be the overall best indicator of stress in a group of animals.