Summer heat stress has a major impact on productivity of beef cattle found in many regions of the United States by a direct increase in core body temperature which, in turn, lowers performance. There is a need for models which provide a more realistic prediction of change in thermal status during summer months. Few controlled studies have examined the dynamic changes in body temperature during exposure to cyclical heat stress conditions. Even fewer studies simulate normal fluctuation in ambient temperature (Ta) to determine the impact of a progressive increase in heat stress on thermal status. The present study was conducted to determine the effect of successively higher levels of heat challenge on thermal status of cattle. Recent studies from our lab have shown that continuous exposure to constant heat challenge (31±1.0C and 50%RH) in a climatic chamber increases core body temperature above the thermoneutral level (TN), particularly during the middle of the night. Therefore, the present experiment was designed to investigate the effect of a progressive increase in Ta on body temperature control. Angus steers (323±5.5 kg average BW; n=12) were housed in the Brody Climatology Laboratory chambers at the University of Missouri-Columbia Chamber and exposed to 18.4±0.0C and 53.47±0.1 RH (TN) for 3 weeks. Chamber Ta was step-increased after this time to an initial heat stress level (HS1) that reached a high point of 33C (1200 - 1700h) and cooled down to 26C during the night (2300-0500h). After 10 days at this Ta, the maximum daily Ta was increased to 36C (50%RH) for 5 days (HS2). Night low Ta remained at 26C during this period. Animals were fed twice a day at 0700 and 1600 and water was available ad libitum. Two weeks prior to the initiation of the study, implanted telemetric temperature transmitters (CowTemp, Model BV-010) were placed in the peritoneal cavity to continuously monitor core body temperature (Tcore) at 20-minutes interval. Respiratory rate and skin surface temperatures of ear, shoulder, rump and tail were recorded four times a day at 0600, 1100, 1600 and 2100. Feed intake was measured daily. Results are being analyzed to determine both initial and long-term responses to progressively higher heat challenges under controlled conditions.