Atrial tachyarrhythmias and atrial pacing are associated with increased cardiac secretion of atrial natriuretic peptide (ANP) in man. Using treadmill exercise to exhaustion, we have studied the effect of exercise induced tachycardia on plasma immunoreactive ANP (IR-ANP) and vasoactive hormones in 6 normal men before and after 6 days of sodium loading (salt supplements and 0.4 mg 9 alpha fludro hydrocortisone daily for 4 days). Similar increases in heart rate and plasma catecholamine levels occurred during exercise in both studies. Sodium loading increased resting supine plasma IR-ANP (P less than 0.037) and suppressed plasma renin and aldosterone, including the renin-aldosterone response to exercise. Plasma IR-ANP increased more than 3-fold during exercise to 48 +/- 7 before and 66 +/- 12 pmol/l after sodium loading (P greater than 0.1). When the response of individual subjects was examined, there was no significant correlation between change in plasma IR-ANP and change in heart rate or catecholamine levels in either exercise study. Exercise induces greater increments in plasma IR-ANP than either acute or chronic sodium loading in normal men and may be a useful and rapid means of assessing the heart's ability to secrete ANP.