We investigated the antioxidant preventive effect of betaine on isoprenaline-induced myocardial infarction in male albino rats. Isoprenaline induced myocardial infarction was manifested by a moderate elevation in the levels of diagnostic marker enzymes (alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase and creatine phosphokinase) and homocysteine in plasma of experimental rats. Significant rise in the level of lipid peroxidation with a concomitant decline in the levels of myocardial non-enzymic (reduced glutathione) and enzymic antioxidants (glutathione peroxidase, glutathione-S-transferase, catalase and superoxide dismutase) was also observed. Oral pretreatment with betaine significantly prevented isoprenaline-induced alterations in the levels of diagnostic marker enzymes and homocysteine in plasma of experimental groups of rats. It counteracted the isoprenaline-induced lipid peroxidation and maintained the myocardial antioxidant defense system at near normal. Histopathological observations also confirmed the protective effect of betaine against isoprenaline-induced myocardial infarction. The results of the present investigation indicate that the protective effect of betaine is probably related to its ability to strengthen the myocardial membrane by its membrane stabilizing action or to a counteraction of free radicals by its antioxidant property.