Approximately 20 years ago a diet high in lard and sucrose was described that produced extensive cardiovascular damage in adult mice. Atrial thrombosis, myocardial necrosis and sudden death were frequent. These experiments were repeated as closely as possible; the adverse effects were prevented by a drinking solution containing 10 micrograms copper/ml. Lack of copper also was associated with anemia, cardiac enlargement and abnormal electrocardiograms. Bradycardia, coupled beats, ectopic ventricular foci, premature atrial beats and prolonged PR interval were found. Lack of copper had no effect on cholesterol in plasma. The results may be germane to ischemic heart disease and the thrombotic susceptibility of women who use oral contraceptives or are pregnant frequently, because copper metabolism is altered in these conditions.