Although plasma brain natriuretic peptide (BNP) levels have been widely measured in patients with acute myocardial infarction (AMI) it is uncertain whether the late thrombolytic therapy (Streptokinase) modulates the levels and whether the levels can predict left ventricular function. The study was designed to elucidate these issues. In 74 patients with AMI, plasma BNP levels were measured at 24 hours after admission. The patients were divided into two groups according to their time of presentation:- group 1 (comprised 31 patients receiving early streptokinase therapy within 6 hours) and group 2 (comprised 43 patients receiving late streptokinase therapy after 6 hours up to 24 hours). In (group 1), the BNP level increased at 24 hours after admission and in group 2 ; the level increased more with the value at 24 hours significantly higher than those in (group 1) (118.8 + or-107.9 pg/ml, versus 287.5 + or-343.1 pg/ml respectively, p= 0.010). Left ventricular function was inversely correlated with the BNP level at 24 hours after admission. Plasma BNP levels can predict left ventricular function. In addition, the importance of early thrombolytic therapy than late therapy may also be supported with BNP kinetics.