We combined hypothalamic tissue and plasma determinations of norepinephrine, dihydroxyphenylalanine, and dihydroxyphenylglycol with measurements of abdominal fat in voluntary running rats to examine the relationship among exercise training, hypothalamic and sympathetic nervous function, and body fat stores. The hypothalamic concentrations of norepinephrine, dihydroxyphenylalanine, and dihydroxyphenylglycol were reduced after exercise training ( P < 0.01), with the amount of norepinephrine being strongly associated with the plasma norepinephrine ( r = 0.58, P < 0.05) and dihydroxyphenylglycol ( r = 0.65, P = 0.01) concentrations. Exercise training resulted in a diminution in abdominal fat mass ( P < 0.01). A strong relationship existed between fat mass and hypothalamic norepinephrine content ( r = 0.83, P < 0.001). The presence of a positive relationship between the arterial and hypothalamic norepinephrine levels provides presumptive evidence of an association between noradrenergic neuronal activity of the hypothalamus and sympathetic nervous function. The observation that abdominal fat mass is linked with norepinephrine in the hypothalamus raises the possibility that alterations in body fat stores provide an afferent signal linking hypothalamic function and the activity of the sympathetic nervous system.