The cardiovascular and metabolic actions of norepinephrine (NE} and their interrelationships were studied at normal room temperature in anesthetized, warm-acclimatized (W-A) (26° +/- 1° C) and cold-acclimatized (C-A) (3° +/- 1° C} rats. Norepinephrine caused a greater increase in the cardiac output, heart rate, stroke volume and right atrial pressure in the C-A animals than it did in the W-A animals. During the early metabolic response to NE (i. e. up to 25% increase in O2 consumption) there was a marked increase in the arterial pressure of both W-A and C-A rats, with the latter showing the greater maximum response. Beyond the 25% level of increased metabolism the arterial pressure and concomitantly the systemic resistance of the C-A animals declined sharply to the preinfusion levels, where they remained throughout the course of infusion. In contrast, the arterial pressure and systemic resistance of the W-A animals remained high. It was concluded that norepinephrine calorigenesis in the C-A rat is supported by an increased cardiac reserve capacity and an ability to preferentially reduce the systemic resistance to actively metabolizing areas (i.e. the viscera}.