BACKGROUND Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in clinical practice, affecting many millions of people world-wide. The treatment of AF has improved significantly during the past decade, but most patients suffer from symptoms and recurrences of arrhythmia. Studies on the neurohormonal remodelling in patients with AF are becoming increasingly important because they may improve the treatment of AF. OBJECTIVE To determine plasma atrial natriuretic peptide (ANP) concentrations in patients with persistent AF, before and one day after electrical cardioversion of AF. METHODS We attempted cardioversion in 42 consecutive patients (30 men and 12 women), aged 57+/-8 years, with persistent nonvalvular AF of 7+/-6 month duration. The underlying heart disease was systemic hypertension in 20, ischaemic heart disease in 17, dilated cardiomyopathy in 3, and lone AF in 2 patients. All patients had left ventricular ejection fraction >45%. The control group comprised 11 subjects with sinus rhythm and no history of AF who were age-, gender- and concomitant disease-matched with the AF patients. Plasma samples of ANP were obtained at rest the day before and 24 hours after cardioversion. RESULTS Cardioversion was successful in 35 patients. Patients with successful or non-successful cardioversion had similar clinical characteristics. The mean baseline ANP level before cardioversion was 59.5+/-15.6 pg/ml and was significantly higher than in the control group - 34.3+/-10.2 pg/ml (p<0.001). In patients in whom sinus rhythm was restored, a significant decrease in the ANP level was observed (59.4+/-16.6 versus 31.4+/-15.0 pg/ml; p<0.01) whereas it did not change in patients with ineffective cardioversion (59.4+/-10.7 versus 60.2+/-10.7 pg/ml, NS). CONCLUSIONS Plasma ANP concentration in patients with AF was significantly reduced after successful cardioversion and remained stable in those with unsuccessful cardioversion.