Previous studies have suggested the presence, in hypothalamic obesity, of an impairment of the energy-dissipating capacity of brown adipose tissue ascribed to a functional disconnection of the sympathetic innervation of this tissue. The following observations demonstrate, with electrophysiological techniques, the presence of a functional link between the ventromedial hypothalamic (VMH) area and the interscapular brown adipose tissue (IBAT) in the rat: the spontaneous activity of the efferent sympathetic nerves reaching the IBAT of normal rats was increased in response to an acute cold stimulus, whereas this increase failed to occur in nerves of VMH-lesioned rats studied 4-7 days after the lesions; and the spontaneous activity of the efferent sympathetic nerves of IBAT decreased rapidly (by greater than or equal to 80% within 30 min) after acute lesions of the VMH area. It is suggested that the VMH area plays a role in increasing the activity of the efferent sympathetic nerves of IBAT during an acute cold stimulus and that alone or in relationship with other, as yet undetermined, central nervous system sites, it has a tonic stimulatory effect on the final common pathways that innervate the IBAT via the efferent sympathetic nerves.