Spectral analysis of fluctuations in heart rate (HR) and arterial blood pressure (BP) during a 6-h infusion of epinephrine (15 ng.kg-1.min-1) or norepinephrine (30 ng.kg-1.min-1) in 10 normotensive males was used to analyze effects of peripheral sympathetic nervous system activity and adrenal medullary discharge on cardiovascular variability. Power spectra were calculated for each 5-min period for HR, systolic BP, and diastolic BP to yield power values for three frequency bands: low (0.02-0.06 Hz), mid (0.07-0.14 Hz), and high (0.15-0.40 Hz). Infusion of epinephrine and norepinephrine induced plasma concentrations of epinephrine and norepinephrine, respectively, within the high physiological range. Spectral analysis showed that low-frequency fluctuations of BP during infusions of epinephrine and midfrequency fluctuations of BP during infusion of norepinephrine changed in opposite directions. These fluctuations may represent different components of short-term cardiovascular control mechanisms during situations that mimic increased sympathoadrenal activity. No changes were observed in HR fluctuations or high-frequency fluctuations of BP after either catecholamine. Our data imply that changes in concentrations of circulating catecholamines cannot be unequivocally labeled as indexes of an altered sympathoadrenal involvement in short-term cardiovascular control.