Changes in the function of the adrenergic neurons of the heart may be important indicators of the adaptations of an animal to physiologic stress and disease. Rates of loss of norepinephrine (NE) from the heart were considered to be proportional to NE secretion and to adrenergic function. In rat hearts, yohimbine induced almost identical increases in rates of loss of 3H-NE and of 125I-metaiodobenzylguanidine (MIBG), a functional analog of NE. Clonidine induced decreases in rates of loss of 3H-NE that were also mimicked by those of 125I-MIBG. In the dog heart, pharmacologically-induced increases and decreases in rates of loss of 123I-MIBG could be measured externally; these values were similar to those obtained for 125I-MIBG in the rat heart. Thus acute changes in the adrenergic neuron activity can be measured in the living heart. The method is applicable to man in determining the capacity of the adrenergic system to respond to provocative challenges.