The objective of this study is to determine the effects of heat, and heat and high relative humidity (RH) on the thermal and cardiorespiratory responses to exercise and recovery. Five Thoroughbred horses (age 3 to 6) completed exercise tests under each of 3 environmental conditions: cool, dry (CD, room temperature (T) = 20°C, RH = 45–55%), hot, dry (HD, T = 32–34°C, RH = 45–55%) and hot, humid (HH, T = 32–34°C, RH = 80–85%). Horses were exercised at a workload equal to 50% of V̇O_2max on a treadmill set at a 10% slope until attainment of a pulmonary artery blood (PA) temperature of 41.5°C followed by a 30 min walking recovery (0% slope), and a further 30 min standing recovery in the same environmental conditions. Blood (PA), rectal, skin (dorsal aspect of the thorax) and muscle (middle gluteal muscle) temperatures and heart rate were measured before, during and after exercise. Respiratory rate was measured before exercise and during the 60 min recovery period. Exercise duration for HD (mean ± s.e. 28 ± 2 min) and HH (16.5 ± 1 min) was significantly (P<0.05) decreased when compared with CD (37 ± 2 min). The rate of increase in PA blood temperature was significantly higher in HH (0.26 ± 0.03°C/min) than in HD (0.17 ± 0.04°C/min) and CD (0.12 ± 0.05°C/min). Temperature in the middle gluteal muscle after 15 min of exercise was significantly higher in HH (41.9 ± 0.3°C) than in HD (40.7 ± 0.25°C) and CD (40.15 ± 0.35°C); whereas rectal temperature at the end of exercise was significantly lower in HH (39.1 ± 0.3°C) than in HD (40.1 ± 0.3°C) and CD (40 ± 0.2°C). The PA blood:skin temperature difference was significantly smaller in HD and HH than in CD. When compared with CD, temperatures at all sites were higher in HD and HH during the 60 min of recovery. Throughout exercise and recovery, heart rate was significantly higher in HH when compared with the other conditions. Post exercise respiratory rate was significantly higher in HD and HH than in CD throughout recovery. It was concluded that the added thermal loads of high temperature and relative humidity increased the rate of heat storage during exercise and delayed dissipation of heat during recovery. The impairment to heat dissipation was probably the result of a reduced capacity for heat transfer from the skin to the environment.