The protective effects of tetrahydroprotoberberines (THPB), viz., l-tetrahydropalmatine (THP), l-stepholidine (SPD), and tetrahydroberberine (THB) on experimental myocardial infarction by ligating the left coronary artery were estimated in rats. The myocardial infarction size (MIS) was determined by nitro-blue tetrazolium technique. THP, SPD, and THB, as well as propranolol, played a remarkable role in diminishing the MIS within 24 h and decreasing the rise of creatine kinase (CK) and aspartate aminotransferase (AST) in the serum within 4 h after the ligation of the coronary artery. Among these drugs, SPD provided the myocardium with the best protective benefit. Systolic and diastolic blood pressures (SBP and DBP) were rapidly lowered after SPD 2.5 mg.kg-1 iv by 39.5% and 48.5%, respectively. The value of the MBP x HR x LVET was concomitantly decreased by 35.1% in the anaesthetized rats. The myocardial contractility and diastolic compliance were not implicated during the experimental regimen. The prophylactic administration of SPD 2.5 mg.kg-1 improved the cardiac hemodynamic alterations caused by ligating the left coronary artery. SPD depressed the elevation of T and LVEDP, and reduction of +dP/dtmax, Vpm, and Vmax besides complete resistance to the reduction of -dP/dtmax and LVSP, underlying the precautions against the damage of the myocardial contractility and diastolic compliance, especially the latter.