This study was undertaken to test whether the presence or absence of a functioning adrenal medulla influences the blood pressure response of conscious normotensive rats to a 30 min infusion of vasodilating agents such as atriopeptin III (1 microgram/min i.v.), a synthetic atrial natriuretic peptide, and sodium nitroprusside (8 micrograms/min i.v.). Rats either had their adrenal medulla removed or were sham-operated 8-10 days before the experimental day. The blood pressure lowering effect of sodium nitroprusside, but not that of atriopeptin III, was enhanced in rats with no adrenal medulla.