The hemodynamic responses of the sympathetic nervous system to the Valsalva maneuver were related to changes in circulating levels of catecholamines, aldosterone and plasma renin activity. Fourteen healthy normotensives (aged 27 +/- 8 years) took part. A catheter was inserted in the forearm then the subject was rested quietly (supine) for 30 mins. The Valsalva maneuver was performed (duration 40 s, intrathoracic pressure 40 mmHg) with continuous recording of supine heart rate. Blood was sampled before the maneuver (basal state) and at the bradycardic post maneuver phase for measurement of plasma noradrenaline, adrenaline, renin activity and aldosterone. In six subjects the procedure was repeated for durations of 10, 20, 30 and 40 s with a 30-min rest between each maneuver. Plasma catecholamines increased consistently (P less than 0.001) from pre- to post bradycardic phases of the maneuver. No changes in plasma renin activity or aldosterone were observed. The maximum tachycardia observed during each maneuver and the increments in catecholamine concentrations were each linearly related to the duration of straining but there was no overall correlation between the tachycardia and catecholamine concentrations. In conclusion under controlled conditions, plasma catecholamine concentrations can be useful indices of the stimulation of the sympathetic nervous system; the Valsalva maneuver does not appear to affect significantly the peripheral renin-angiotensin system; and the heart rate response to the Valsalva maneuver does not appear to be mediated solely by the sympathetic nervous system.