Heat stress, caused by excessive heat load, can impact on animal production. Although animals are able to adapt to hot climates, their adaptive mechanisms may be detrimental to their productive performance. Substantial reductions in productivity and even deaths may occur when cattle in feedlots do not have the opportunity to reduce their heat load which usually occurs during the night when Ta are lower than during the day time. The studies reported in this dissertation were undertaken to evaluate the impact of heat stress on the physiology, rumen fermentation and rumen microbiota of grain-fed cattle that were subjected to high heat load. Seventy-two Angus (Bos taurus) yearling steers were used in two experiments, each of which involved two treatments [control group (CON) and heat stress group (HOT)] with 3 replications of 12 steers per treatment. The experiments were undertaken in the Climate