Physiological data were collected during two 3-day-event competitions: one (H) held in hot and the other (CL) in cool conditions. During H, ambient temperature and relative humidity were 2.5 degrees C-35 degrees C and 74-36% respectively, while during CL, ambient temperature and relative humidity were 7.8 degrees C-8.3 degrees C and 46%-41%, respectively. Rectal temperature, heart and respiratory rates were recorded on arrival at the event, at the end of Phase C and 6 min later, at the end of Phase D and for 30 min at 10 min intervals after each horse finished Phase D (Recovery Period). Because of the heat, the rest-pause during the Endurance Test was extended from 10 to 15 min for horses competing in H, and horses at H were aggressively cooled by repetitive bathing with ice water during the rest-pause and the 30 min Recovery Period. Heart rate was significantly higher (P < 0.05) at the end of Phase C in horses participating at H, as compared to those participating at CL. Heart rates were significantly decreased in both groups after 6 min in the rest-pause and by 10 min after the finish of Phase D. Rectal temperature were significantly higher in horses competing at H than in those competing at CL at the end of Phase C and 6 min later, and at 10 and 20 min after the finish of Phase D. In both groups, rectal temperatures decreased significantly during the first 6 min in the rest-pause and at 10 and 20 min after the finish of Phase D. Fifty-five of 79 (69.6%) horses starting Phase A at H completed Phase D, as compared to 23 of 28 (82.1%) of starters at CL (P > 0.05). Of 10 horses eliminated during the rest-pause at H, 3 were lame, 1 had exertional rhabdomyolysis, 4 were exhausted and 2 were lame and exhausted. Two horses were eliminated during the rest-pause at CL:1 was lame and the other had exertional rhabdomyolysis. There was marked individual variation in horses' responses to heat at H. Heat may have limited the effectiveness of evaporative cooling in horses at H, and resultant thermal stress probably contributed to exhaustion in horses unable to complete the Endurance Test. Aggressive cooling during the rest-pause and the Recovery Period was important to facilitate heat dissipation in horses at H. These measures were not necessary at CL.