It is generally agreed that the site of heat production during nonshivering thermogenesis is the brown adipose tissue (BAT) and that the triggering event for heat production is the interaction of noradrenaline (NA) with its receptor on the plasma membrane. Following this initial event, several changes occur which result in increased rates of cAMP synthesis, redistribution of ions across the membrane, enhanced rates of lipolysis, and increased mitochondrial oxidation of substrates. BAT is also a target for the anabolic effect of insulin. Available evidence shows that insulin receptors are present on the BAT plasma membrane and that insulin can oppose the metabolic effects of catecholamine on BAT. We have studied more particularly the response of BAT adenylate cyclase to catecholamines in an animal model (the ob/ob mouse) which has a defective thermogenic response. The capacity of adenylate cyclase to be stimulated by catecholamines was significantly less in the tissue of obese mice than in lean controls. To produce a response equal to the half-maximal response in the lean mouse, a 10-fold increase in the NA concentration was required in the BAT of the obese mouse. These results are in harmony with those of others showing that the lipolytic response to catecholamines is abnormal in the BAT of the obese mouse. The adenylate cyclase activity can be altered by changes in the lipid composition of the diet and by manipulation of hormone levels. It is likely that the alteration in adenylate cyclase responsiveness is one of the contributing factors in the impaired thermogenesis and obesity in this animal.