Myocardial infarction was produced in dogs, and the changes in nucleic acid synthetic activity were investigated quantitatively by microspectrophotometer in the myocardial cells as time progressed. DNA value, immediately after infarction, was greatly increased in comparison to that of the control group. At two weeks after infarction the value had increased to the highest level. After this point the value decreased, and, in 12 weeks, the mean value was back to the control level. Changes in RNA followed a pattern similar to DNA changes. The mechanism of the repair process of myocardial infarction was investigated.