Barbital-sedated, warm-acclimated (WA) or cold-acclimated (CA) rats were infused intravenously with noradrenaline (NA) at doses that elicited graded calorigenic responses. Blood flow (Q) to the various bodies of brown adipose tissue (BAT), the major sites of the NA-induced calorigenesis, was measured with labeled microspheres. The O2 content of arterial blood and of venous blood from interscapular BAT and the concentration of NA in arterial plasma (ANA) were also determined. ANA was linearly related to the dose of NA. Calorigenic response and the Q of total BAT and of separate bodies of BAT were sigmoid functions of ANA. The threshold for calorigenic response or for increased flow to BAT was an ANA of about 2 ng/mL (12 nM), except for some bodies of BAT in CA rats where it was closer in 4 ng/mL. Delivery of O2 to total BAT and calorigenic response were related linearly. The bodies of BAT were heterogeneous in Q per gram in CA rats the hierarchy in Q per gram changed markedly as ANA and calorigenic response increased. The analysis of these results takes into account that calorigenesis in BAT normally is not mediated by circulating NA, that in NA-infused rats neuronal and extraneuronal uptakes of NA would effect a lower concentration of NA at the adrenoceptors of BAT than in circulation, and that many factors such as organization and density of adrenergic innervation and the number of efficacy of receptors must have contributed to determining the measured responses of BAT. It is concluded that the differently located bodies of BAT in rats may have significant differences in composition and structure and that they may undergo differential development during cold acclimation.