AbstractYoung adult mice of an albino stock (Taconic Swiss) were fed a hypolipotropic diet with a fat content (lard) of 28% and 8% protein (casien). The same diet containing betaine hydrochloride (2%) as a lipotropic supplement was fed to one‐half the mice.After seven weeks of restriction to these diets (with and without betaine supplementation) pronounced lesions had developed in the hearts. These lesions consisted of myocardial necrosis and thrombi within atrial lumina. The thrombi increased progressively in size, reaching critical and lethal dimensions for 75% of the animals by 13 weeks. Atrial dilatation, as a compensating mechanism, was a typical response. Small foci of myocardial necrosis with calcification and fibrosis were common in the ventricles. All mice surviving for longer than 13 weeks showed the myocardial changes ventricles. All mice surviving for longer than 13 weeks showed the myocardial changes and the thrombosis described above. Mural necrosis was observed in the intramyocardial branches of coronary arteries but thrombosis was infreqent within these vessels. There was extreme weight loss which paralleled the onset and increasing severity of the cardiac lesions. Betaine, as a dietary lipotrope, did not prevent or alter the cardiovascular lesion