We have evaluated the preventive effects of an aqueous Aegle marmelos leaf extract (AMLEt) in isoprenaline (isoproterenol)-induced myocardial infarction in rats. Rats were pretreated with AMLEt (50, 100 or 200 mg kg(-1)) for 35 days. After the treatment period, isoprenaline (200 mg kg(-1)) was administered subcutaneously to rats at an interval of 24 h for two days. The activity of creatine kinase (CK) and lactate dehydrogenase (LDH) was significantly increased in serum and significantly decreased in heart of isoprenaline-treated rats. Pretreatment with AMLEt decreased the activity of CK and LDH in serum and increased them in the heart. The activity of sodium-potassium dependent adenosine triphosphatase (Na(+)K(+)ATPase) was significantly decreased while the activity of calcium dependent adenosine triphosphatase (Ca(2+)ATPase) was simultaneously increased in the heart and aorta. AMLEt pretreatment increased the activity of Na(+)K(+) ATPase and decreased the activity of Ca(2+)ATPase in the heart and aorta simultaneously. The levels of cholesterol and triglycerides increased, while the levels of phospholipids decreased in the heart and aorta of isoprenaline-treated rats. In AMLEt-pretreated rats the levels of cholesterol and triglycerides decreased whereas phospholipids increased in heart and aorta. All the deranged biochemical parameters were restored with 200 mg kg(-1) AMLEt. Similarly alpha-tocopherol (60 mg kg(-1))-pretreatment to isoprenaline-treated rats exhibited a significant effect on all the parameters studied. The results from this study may have clinical relevance.