We prospectively determined the time course of recovery of atrial mechanical and endocrine function in patients following DC cardioversion for persistent atrial fibrillation (AF). Twenty-three consecutive patients underwent successful DC cardioversion (mean age 64 years, 20 male). By 28 days, nine had reverted to atrial fibrillation. Recovery of atrial mechanical (peak A wave velocity) and endocrine function (atrial natriuretic peptide, ANP) were assessed at four time points: immediately pre-cardioversion, and then 4 h, 7 and 28 days post. The 14 patients maintaining sinus rhythm formed the success group. In this group, peak A wave velocity increased significantly over time from 0.28±0.03 m/s (mean±S.E.M.) at 4 h to 0.50±0.03 m/s at day 7 and 0.67±0.05 m/s at day 28 (p<0.001). Plasma ANP was 150±34 pg/ml immediately prior to DC cardioversion in the success group. This showed an initial dip at 4 h to 44±9 pg/ml (p<0.001). By day 7, plasma ANP had increased to 105±21 pg/ml (p<0.05 vs. baseline and 4 h) and then appeared to remain constant, being 102±19 pg/ml at day 28 (p=0.06 vs. baseline). A similar early reduction in ANP levels was seen in the group who subsequently reverted to atrial fibrillation. Baseline ANP levels did not predict subsequent successful maintenance of sinus rhythm. Initial “stunning” in both atrial mechanical and endocrine function occurs in patients following DC cardioversion for atrial fibrillation. Whilst endocrine function appears to fully recover by day 7, mechanical function continues to improve beyond day 7.