Abstract 1. 1. On 2 resting animals 45 experiments with 120 periods were performed in a warm and dry environment ( T a = + 34°C, r.h. = 22%). 2. 2. By means of chronically-implanted intravascular heat exchangers body-core temperature was kept constant at different values between 37.5 and 42.5°C, which caused heat production (M) to level between 1.5 and 9 W kg −1 and respiratory evaporative heat loss (REHL) between 0.5 and 3 W kg −1. 3. 3. During internal cold stress a 200% increase in M was associated with: (a) cardiac output to increase by 50%; (b) haemoglobin concentration to increase by 20%; and (c) A V O 2 difference to increase by 100%. Mean arterial pressure (MAP) and stroke volume did not consistently change. 4. 4. During internal heat stress cardiac output (CO) did not rise by more than 30% even when body temperature reached 42.5°C and REHL attained peak values of 3 W kg −1. MAP remained either constant or increased slightly. 5. 5. It is concluded that goats during cold stress primarily increase and utilize the gas transport capacity of the blood rather than increase CO. During severe internal heat stress, maximal activity of heat-dissipating mechanisms does not require more than a limited increase in cardiac output. Under both conditions the redistribution of CO appears to play an important role.