The present study was performed to evaluate the role of different doses of garlic oil (GO) and its constituent diallyl disulfide (DADS) as cardioprotective when concurrently administered with atenolol (ATN) during isoproterenol (ISO) induced myocardial dysfunction in rats. Sprague dawley rats (150-250 g) were administered GO (50 and 100 mg/kg), DADS (4.47/8.94 mg/kg), ATN (6 mg/kg) and GO, DADS in combination with ATN every alternate day for three weeks, in their respective groups. At the end of treatment, two doses of ISO were administered (150 mg/kg. sc) for two consecutive days. A change in biomarkers and Antioxidants levels reflect the influence of prophylactic treatment. Electrocardiographic (ECG) changes were restored to normal in all treated groups compared to ISO group. GO and DADS caused significant fall in LDH and CK-MB activities in serum with simultaneous rise in heart tissue homogenate (HTH). The biomarker activities were also decreased in serum of ATN group compared to ISO group. However, there was no significant change in SOD and Catalase levels in ATN alone groups. The endogenous antioxidant levels were significantly increased in GO, DADS and their combination with ATN groups. TBARS levels were significantly decline in all treated groups compared to ISO group. Among the different treated groups, low dose of GO (50 mg/kg) with ATN and high dose of DADS (8.94 mg/kg) with ATN was more effective in ameliorating the ISO induced myocardial damages. From the above results and histopathalogical examination it was concluded that DADS plays a major role in ameliorating the ISO induced myocardial damages.