In eight patients with atrial fibrillation of less than 3 months duration and without congestive heart failure the plasma concentration of atrial natriuretic peptide was determined one day before, the day after and again 30 days after electrical cardioversion therapy. The pretreatment plasma concentration of the peptide was 99 pg mg-1 (23-480, median and range). The day after cardioversion to sinus rhythm the peptide concentration had normalized to 36 pg ml-1 (18-151). The plasma concentration of atrial natriuretic peptide remained stable in all but one patient for a period of 30 days (46 pg ml-1, 16-695) (P = 0.03). In conclusion, the plasma concentration of atrial natriuretic peptide in patients with atrial fibrillation was significantly reduced after electrical cardioversion to sinus rhythm and remained stable for a period of 30 days.