The relative importance of sympathetic nerve (SNS) activity and adrenal medullary secretion in various physiological situations has generally been inferred from measurements of norepinephrine (NE) and epinephrine (E), respectively, in urine or plasma. Increasing evidence, however, indicates that under certain conditions the adrenal medulla may release substantial amounts of NE as well as E. In several of these circumstances, estimates of SNS activity based on the measurement of NE turnover in peripheral tissues of experimental animals indicate diminished SNS function, a reduction that is independent of adrenal medullary secretion. These reciprocal alterations in SNS and adrenal medullary activity fall into two patterns. First, when SNS activity is suppressed by fasting, adrenal medullary responses to various stimuli are enhanced. Second, for certain stimuli the SNS response is biphasic, with an initial suppression followed by subsequent stimulation; during the first phase adrenal medullary secretion is markedly increased. The physiological contribution of the adrenal medulla, therefore, would be particularly important under conditions of SNS suppression.