The effect of synthetic alpha human atrial natriuretic peptide on catecholamine release from human pheochromocytomas was studied both in vivo and in vitro. Iv infusion of atrial natriuretic peptide at a rate of 0.1 microgram.kg-1.min-1 for 60 min into two normotensive patients with pheochromocytoma caused a small decrease in the mean blood pressure, increase in the heart rate, and marked increase in the plasma level of norepinephrine (2.08 to 6.83 nmol/l, and 1.15 to 2.83 nmol/l, respectively) compared with 0.60 +/- 0.10 to 1.19 +/- 0.20 nmol/l in normal subjects. Treatment with atrial natriuretic peptide also increased the plasma epinephrine level from 0.34 to 1.27 nmol/l, and from 0.67 to 0.79 nmol/l in the patients with pheochromocytoma, but not in the normal subjects (0.05 +/- 0.01 to 0.05 +/- 0.01 nmol/l). After removal of the tumour, the responses of the plasma norepinephrine and epinephrine to atrial natriuretic peptide infusion were normalized. There was no significant effect of 10(-8) to 10(-5) mol/l atrial natriuretic peptide on the basal release of catecholamines from isolated superfused pheochromocytoma tissue. Atrial natriuretic peptide (10(-7) mol/l) did not affect the increase in catecholamine release induced by glucagon (10(-5) mol/l). These results suggest that the exaggerated responses of plasma catecholamines to atrial natriuretic peptide in patients with pheochromocytoma may be due to a washout effect resulting from change in blood flow in the vessels feeding the tumour rather than increased sympathetic nerve activity induced by hypotension and hypovolemia. The results also suggest that atrial natriuretic peptide dose not have any direct action on pheochromocytoma tissue causing catecholamine release.