OBJECTIVE The aim of this study was to evaluate and compare the effects of physiologic and pharmacologic sympathetic stimulation on time and frequency domain indexes of heart rate variability. BACKGROUND Measurements of heart rate variability have been used as indexes of sympathetic tone. To date, the effects of circulating catecholamines on heart rate variability have not been evaluated. METHODS Fourteen normal subjects (eight men, six women, mean [+/- SD] age 28.5 +/- 4.8 years) were evaluated. Five-minute electrocardiographic recordings were obtained in triplicate after physiologic and pharmacologic sympathetic stimulation: during upright tilt, after maximal exercise, during epinephrine and isoproterenol infusions at 50 ng/kg body weight per min, during beta-adrenergic blockade and during combined beta-adrenergic and parasympathetic blockade. RESULTS Beta-adrenergic stimulation resulted in a significant decrease in time domain measures of heart rate variability. The frequency domain indexes showed variable responses, depending on the individual stimulus. Tilt caused an increase in low frequency power and in the ratio of low to high frequency power. These changes were not necessarily observed with other conditions of beta-adrenergic stimulation. Double blockade suppressed baseline heart rate variability, but beta-adrenergic blockade had no significant effect. Time domain measures of heart rate variability demonstrated excellent reproducibility over the three recordings, but the frequency domain variables demonstrated fair to excellent reproducibility. CONCLUSIONS These findings suggest that different modes of beta-adrenergic stimulation may result in divergent heart rate variability responses. Thus, current heart rate variability techniques cannot be used as general indexes of "sympathetic" tone. Studies utilizing heart rate variability to quantify sympathetic tone need to consider this.