Injection of CRF-41 (2-5 nmol) into the third ventricle, or the paraventricular nucleus of anaesthetized rats caused a marked rise in the temperature of the interscapular brown adipose tissue (BAT) depot (peak rise 0.8-1 degree C) which was inhibited by prior intravenous injection of propranolol. There was also a significant increase in the mitochondrial proton conductance pathway of brown adipose tissue, assessed from the binding of guanosine diphosphate (GDP) to mitochondria isolated from the interscapular (89% above control) and perirenal and para-aortic depots (130%). Acute surgical sympathectomy of interscapular brown adipose tissue immediately prior to injection of CRF significantly attenuated the increase in mitochondrial GDP-binding. Hypophysectomized (HYPX) rats showed a large (180%) increase in GDP-binding of brown adipose tissue 7 days after surgery and this was almost completely prevented by denervation of the interscapular depot prior to hypophysectomy. Acute injection of morphine also reduced the GDP-binding in hypophysectomized, but not in control rats. These data demonstrate that central injection of CRF stimulates thermogenesis in brown adipose tissue, probably by modifying sympathetic outflow. The activation of brown adipose tissue following hypophysectomy was also dependent on the sympathetic innervation and could be due to an increase in release of CRF.