Abstract Studies have been made on the interscapular brown fat of normal and arousing hamsters in vivo during the altered metabolism induced by hypoxia or infusion of norepinephrine. Ventilation of the lungs with nitrogen caused a prompt fall of the oxygen tension and temperature of the brown fat of normal hamsters. This was followed by an increase of fluorescence, which started when the tissue oxygen tension fell to a critical level. The oxygen tension of brown fat from arousing hamsters was one third that, of normal hamsters, an indication of a more active metabolic state of the tissue during arousal. With arousing hamsters, the fluorescence level increased immediately after N 2 inhalation, and the brown-fat temperature decreased. Norepiucphrmc infusion into the external jugular vein produced a prompt decrease of fluoresence and oxygon tension, and an increase of temperature in the brown fat of normal and arousing hamsters. The pyridine nucleotide fluorescence response is considered to reflect redox changes of the mitochondrial NAD+ system. Since enhanced thermogenesis coincided with an increased flow of electrons through the respiratory chain and an oxidation of the pyridine nueleotides, it is deduced that the mitochondria of brown fal in vivo are capable of oxidative phosphorylation. A theory for the chemical mechanism of thermogenesis is developed which suggests that heat production is controlled by the rate of respiration. Hydrolysis of triglyceride, through norepinephrine mediation, provides the respiratory fuel, while other mechanisms sustain the supply of phosphate acceptor to the mitochondria.