Abstract Accumulating evidence indicates that atrial natriuretic peptide (ANP) is a circulating hormone in humans. 1–4 Plasma levels of ANP increase in patients with various heart diseases such as paroxysmal atrial arrhythmias 2 and congestive heart failure. 3,4 If atrial stretching due to increased atrial pressure stimulates ANP secretion, 4 plasma ANP levels may be altered in patients with bradyarrhythmias and in those with pacemaker syndrome, a major clinical complication in patients in whom artificial pacemakers are implanted. 5 To clarify the role of ANP in the pathophysiology of these states, we examined plasma ANP levels in patients with bradyarrhythmias and in those treated with pacemakers of various modes.