Abstract： Objective To investigate the relationship between the cardiac function and the levels of atrial natriuretic peptide (ANP)in pericardial fluid and plasma in patients with rheumatic heart diseases. Methods The study involved 33 patients undergoing open heart surgery for either heart valvuloplasty or valve replacement. ANP concentrations in the plasma, pericardial fluid, and pericardial tissues were measured by using of radioimmunoassay (RIA). Results The level of ANP in human pericardial fluid was significantly higher than that in the plasma (0.614±0.486 ng/ml vs 0. 451 ±0. 184 ng/ml, P =0. 046). The ANP concentration in pericardial fluid of patients with left ventricular ejection fraction(LVEF) ＜ 50% was much higher than that of patients with LVEF ≥50%. Conclusion The ANP in the pericardial fluid may be involved in the deterioration of cardiac function in patients with rheumatic heart diseases.