The plasma concentration of atrial natriuretic polypeptide (ANP) was measured in nine patients with essential hypertension during two grades of exercise tests performed in the supine position on a bicycle ergometer. The plasma ANP concentration significantly increased from 97.0 ± 19.2 pg/ml to 107.6 ± 23.7 pg/ml ( p < 0.05) during low-grade exercise (50% of the maximal heart rate) and from 96.2 ± 16.5 pg to 192.8 ± 30.7 pg/ml ( p < 0.01) during high-grade exercise (75% of the maximal heart rate). During high-grade exercise plasma epinephrine and norepinephrine concentrations showed significant increases. The plasma ANP concentration was significantly correlated with systolic blood pressure ( r = 0.51; p < 0.05). Patients with essential hypertension showed greater absolute increases in the plasma ANP concentration and systolic blood pressure during exercise compared to normotensive subjects. These results suggest that exercise stimulates secretion of ANP in response to its intensity in patients with essential hypertension and that a greater rise in atrial pressure, resulting from a greater elevation of systolic blood pressure, may be involved in the exaggerated secretion of ANP in patients with essential hypertension.