1. Lipid mobilization during a hypocaloric diet may be enhanced by a pharmacological approach using alpha 2-adrenoceptor antagonists since these drugs are known to increase sympathetic tone and stimulate lipolysis. Studies were undertaken in the dog in order to evaluate the effects of oral yohimbine administration (alpha 2-adrenoceptor antagonist) on heat production, metabolic, endocrinological and cardiovascular parameters. 2. Acute oral yohimbine (0.25 or 0.40 mg kg-1) provoked an increase in plasma non-esterified fatty acids. The drug increased sympathetic nervous system activity as indicated by the increased level of plasma noradrenaline. These effects persisted during the entire experimental period (4 h). The increase in plasma noradrenaline level was two fold higher with the higher dose of yohimbine (0.4 mg kg-1). The plasma adrenaline level was increased only with the higher dose. 3. Yohimbine transiently increased plasma insulin and the effect was dose-dependent. 4. Yohimbine (0.25 mg kg-1) enhanced heart rate and arterial blood pressure. 5. The effect of yohimbine on oxygen consumption, carbon dioxide and heat production was determined by indirect calorimetry. The drug (0.25 mg kg-1) increased O2 consumption and CO2 and heat production 30 min after its administration and the effect persisted over the experimental period. The respiratory quotient, rather low in the fasting animals, remained unchanged. 6. The present work indicates that thermogenesis and lipid mobilization are enhanced during fasting in the dog by alpha 2-adrenoceptor blockade. Yohimbine also induced a transient increase in plasma insulin level and increased heart rate and blood pressure. The lipid mobilization plus the action on thermogenesis observed after yohimbine draw attention to the putative interest of a2-antagonists in the pharmacological treatment of obesity during restricted calorie intake.