Aim To investigate the significance of the changes on plasm brain natriuretic peptide (BNP) in patients with acute myocardial infarction(AMI). Methods 63 patients with AMI were divided into losartan (n=19), enapril(n=23) and control (n=19) group. The plasm levels of BNP was measured in patients with AMI on admission and 24 h as well as on the day of 5 and 14, compared with ten health subjects. Left ventricular end diastolic volume index (LVEDVI) and left ventricular end systolic volume index (LVESVI) were determined by echocardiogram on the day of 5 and 14. Result The plasm levels of BNP was significantly increased on admission in control group compared with that of the health subjects(P<0.01) and increased further on the day of 5 and 14 than that on admission(P<0.05). Enapril and losartan treatment decreased plasm BNP level on the day of 5 and 14. The plasm level of BNP on the day of 5 and 14 in patients with AMI correlated with LVEDVI and LVESVI (r=0.62～0.77,P<0.01). The plasm levels of BNP levels in patients with anterior myocardial infarction (AMI) were higher than those in patients with inferior. MI and were lower in patients with early reperfusion than those with non reperfusion. Plasm levels of BNP on the day of 5 and 14 correlated with plasm peak creatine phosphokinase (r=0.32, 0.67,P<0.05, 0.01). Conclusions The plasm level of BNP increased continuously within 14 days after the onset of AMI, and this would reflect infarction area and infarction expansion post AMI.