Many experimental studies have utilized the activity of dopamine-beta-hydroxylase (DBH) as an index of sympathetic activity, since this enzyme is not submitted to uptake mechanisms or to enzymatic metabolism as are the circulating catecholamines norepinephrine (NE) and epinephrine (E). However, large discrepancies have been found between the results of these studies. The hypothesis which is examined in this study is that these discrepancies might arise from the different intensities in the stimuli utilized. In order to examine this possibility, plasma DBH activity was measured in seven subjects at rest and in three different conditions known to increase sympathetic activity to varying degrees, i.e., during hand-grip, standing, and supra-maximal bicycle exercise. Plasma NE and E concentrations were also measured during these stimuli. The results of this study show that DBH activity increases above resting levels only during supramaximal dynamic exercise, while plasma NE and E concentrations increase during each experimental condition. Furthermore, the increases in NE and E are related to each other and are also related to heart rate when all experimental conditions are considered. On the contrary, the variations in plasma DBH (expressed as a percentage of the resting value) are not related to other sympathetic indices or to heart rate. Since NE and E vary little from subject to subject at rest and increase discretely in response to the various stimuli, these plasma catecholamine concentrations therefore appear to represent the most accurate indices of sympathetic activity.