The role of catecholamines in thermogenesis has been investigated in trained young pigs in various situations which increases heat production. Plasma noradrenaline and adrenaline concentrations were estimated by radioenzymic assay of samples taken from an indwelling jugular catheter to prevent disturbance and stress. 2. During 3 hr at ambient temperatures of between 25 and -5 degrees C, there was curvilinear increase in noradrenaline concentrations as temperature decreased, but adrenaline did not change. After 1-3 days at 5 or 1 degrees C, the concentrations of both amines were similar to those found at thermal neutrality. 3. Cooling the cervical region of the spinal cord resulted in a large increase in plasma noradrenaline whereas local cooling of the hypothalamus was associated with a small increase in the hormone. When cooling either region of the central nervous system, plasma adrenaline concentrations did not change. 4. During exposure to 45 degrees C, rectal temperature increased to 43 degrees C. Concentrations of catecholamines remained low or declined until the rectal temperature was above 41.5 degrees C when the concentrations increased in most animals. 5. Induced physical activity lasting 40 min was associated with an increase in both noradrenaline and adrenaline concentrations. 6. Blood samples from the inferior vena cava, just cephalad to the outlet of the adrenal veins, had much higher concentrations of noradrenaline during cold exposure than those taken simultaneously from the jugular vein. Therefore, at least part of the increase in noradrenaline concentrations reported above may have been due to release of the hormone from the adrenal medulla. 7. It is concluded that during short-term exposure to cold, and in exercise, there is a rapid catecholamine response, producing concentrations in the blood which could be high enough to stimulate thermogenesis. During longer-term cold exposure, catecholamine output is low and other factors must be of greater importance in maintaining thermogenic processes. Further, catecholamine output may result from thermal stimuli not only from outside the external surface of the animal but also from within the central nervous system.
