Lactate dehydrogenase isozyme pattern and activity and pyruvate kinase activity were studied in animals with severe sideropenic anemia. This condition was induced by feeding rats a diet of non-fat dry milk-sucrose from the time of weaning. Anemia was indicated by the lowering of blood hemoglobin levels to 25 and 20% of normal in two groups after 30 and 85 days, respectively. In both anemic groups there was accompanying cardiac enlargement and retardation of body weight gain. Statistically significant increases in cardiac lactate dehydrogenase (25–33%) and pyruvate kinase (25%) activity were observed in both groups relative to controls. An alteration of the normal myocardial lactate dehydrogenase isozyme pattern in the direction of increased lactate dehydrogenase-5 took place in both groups of anemic rats. This resulted from a dramatic increase in the M-lactate dehydrogenase subunit contribution (45–54%) to total lactate dehydrogenase activity.