AbstractExamination of the effect of 2,4‐dinitrophenol (DNP) in vivo on the brown adipose tissue of cold‐exposed rats as well as the effect of DNP and dicumarol in vitro, indicates that brown fat does possess a functional electron transport‐coupled phosphorylating system. Moreover, the fact that a norepinephrineinduced thermogenic response (in vivo) can be elicited from the brown fat after DNP administration implies that the effect of norepinephrine (NE) is not primarily due to stimulation of an adenosine triphosphatase system. Furthermore, since the magnitude of the NE‐stimulated temperature increase is not diminished by prior treatment with DNP, it appears that the effect of NE is not achieved through any significant degree of uncoupling by the released fatty acids. Alternatively, our data suggest that under basal conditions (i.e., when the animal is not stimulated by cold stress or NE) the heat production (oxygen consumption) of the brown fat is limited by the availability of substrate rather than ADP. Conversely, it is proposed that under states of cold stress or NE infusion the thermogenic effect is induced through stimulation of lipolysis and consequent enhancement of substrate accessible for mitochondrial oxidation.