The effects of exercise on the severity of isoproterenol-induced myocardial infarction were studied in female albino rats of 20,40,60 and 80 weeks of age. The rats were trained to swim for a specific duration and for a particular period. The occurrence of infarcts were confirmed by histological methods. Elevations in the serum GOT and GPT were maximum in the sedentary-isoproterenols and minimum in the exercise-controls. These changes in the serum transaminases were associated with corresponding depletions in the cardiac GOT and GPT. However, age was seen to interfere with the responses exhibited by the young and old rats. Studies dealing with myocardial infarction are more informative when dealt with age.