Male and female, arteriosclerotic and nonarteriosclerotic rats were subjected to acute myocardial infarction by two, subcutaneous injections (spaced 24 hr apart) of isoproterenol. During the immediate postinfarct repair phase all of the experimental animals were made severely diabetic with alloxan. Two weeks later the animals were sacrificed and their blood and pertinent organs analyzed for biochemical and pathologic changes. Females survived the myocardial infarct with superimposed diabetes in significantly greater than males. In addition to marked loss in body weight all of the experimental animals developed marked adrenal hypertrophy and thymus gland involution, cardiac hypertrophy, and unusual increase in ovarian or testicular size and weight. The combined conditions of myocardial infarction + diabetes led to substantial increases in serum creatine phosphokinase (CPK) and glutamic oxaloacetic transaminase (SGOT) whereas the enzymes glutamic pyruvic transaminase (SGPT) and lactic dehydrogenase (LDH) were reduced. Although serum triglyceride levels were greatly elevated, total cholesterol and free fatty acids were reduced. All of the animals were severely hyperglycemic and had greatly increased B.U.N. levels. Diabetes caused hypercalcemia but diabetes + myocardial infarction was associated with a definite reduction of this hypercalcemia. Despite marked adrenal hypertrophy, circulating Cmpd. B levels were subnormal. The diabetic condition and its attendant hyperlipidemia did not alter the morphologic nature of the arterial lesions in the breeder rats but the diabetes did cause definite impairment of the usual myocardial repair process observed in these rats with a particularly high incidence of left ventricular aneurysms in males.