Plasma brain natriuretic peptide (BNP) was evaluated before and after sinus rhythm restoration in patients with paroxysmal and persistent atrial fibrillation (AF) who had underlying hypertension or coronary heart disease and normal left ventricle function. Twenty-four hours after successful cardioversion, plasma BNP decreased significantly to levels that had been measured in controlled subjects: from 95 to 28 pg/ml in 24 patients in the paroxysmal AF group and from 75 to 41 pg/ml in 36 patients in the persistent AF group. This indicates that AF affects BNP secretion in patients with AF and that some BNP may be atrially delivered.