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The effects of relative humidity (RH) and high ambient temperature (T) on physiological responses and animal performance were studied using 12 groups (10 gilts per group) in pens inside respiration chambers. The microclimate in the chamber was programmed so that T remained constant within a day. Each day, the T was increased by 2°C from low (16°C) to high (32°C). Relative humidity was kept constant at 50, 65, or 80%. The pigs¿ average initial BW was 61.7 kg (58.0 to 65.5 kg), and their average ending BW was 70.2 kg (65.9 to 74.7 kg). Respiration rate (RR), evaporative water (EW), rectal temperature (RT), skin temperature (ST), voluntary feed intake (VFI), water-to-feed ratio (rW:F), heat production (HP), and ADG were analyzed. The animals had free access to feed and water. We determined the T above which certain animal variables started to change: the so-called inflection point temperature (IPt) or "upper critical temperature." The first indicator of reaction, RR, was in the range from 21.3 to 23.4°C. Rectal temperature was a delayed indicator of heat stress tolerance, with IPt values ranging from 24.6 to 27.1°C. For both these indicators the IPt was least at 80% RH (P <0.05). Heat production and VFI were decreased above IPt of 22.9 and 25.5°C, respectively (P <0.001). For each degree Celsius above IPt, the VFI was decreased by 81, 99, and 106 g/(pig·d) in treatments 50, 65, and 80% RH, respectively. The ADG was greatest at 50% RH (P <0.05). Ambient temperature strongly affects the pigs¿ physiological changes and performance, whereas RH has a relatively minor effect on heat stress in growing pigs; however, the combination of high T and high RH lowered the ADG in pigs. The upper critical temperature can be considered to be the IPt above which VFI decreased and RT then increased. Temperatures of the magnitude of both these IPt are regularly measured in commercial pig houses. We conclude that the upper critical temperatures for 60-kg, group-housed pigs fed ad libitum a