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The effect of simultaneous changes in exercise and ambient temperature on body heat balance and physiological strain were studied. The exercise/rest periods were either 10/10 minutes or 30/30 minutes, and the total duration of the protocol was 120 min. Exercise (walking 6 km/h on treadmill, slope 2°) was performed in cold environment (-15°C, air velocity 2.5 m/s). The resting periods were spent sitting at +10°C, air velocity 0.2 m/s, with the same clothing as during exercise. The subjects were 7 voluntary healthy young men. They were wearing Finnish military winter clothing (M91, thermal insulation about 2 clo) and rucksack (12 kg). During the rest periods drinking of water was allowed ad libitum. Data are given as mean ± SE. The mean skin temperature during the 10/10 schedule was 31.7 ± 0.2°C and during the 30/30 schedule 31.3 ± 0.3°C. Deep body temperature was in average 37.5°C during both schedules. At the end of the last exercise period oxygen consumption was 33.5 ± 0.9 ml/min/kg in the 10/10 schedule and 32.4 ± 3.8 ml/min/kg in the 30/30 schedule. During the exercise periods heart rate was in average 150 beats/min in both schedules. The amount of sweating was greater during the 10/10 schedule (809 ± 118 g) than during the 30/30 schedule (667 ± 182 g). Also the fluid intake was greater during the 10/10 schedule (457 ± 121 g) than during the 30/30 schedule (141 ± 41 g). In conclusion, the body heat balance and physiological strain were comparable in both exercise/rest schedules. The amount of sweating as well as fluid intake were greater during the 10/10 minutes exercise/rest schedule. This finding can be due to the fact that the number of rest periods was greater in the 10/10 schedule, and the transition from exercising in -15°C to resting in +10°C possibly promotes sweating. These findings suggest that specified instructions for clothing and fluid intake are needed for different combinations of exercise and rest in changing ambient temperatures.