We studied on the ralationship between electrocardiographic changes and thiamine contents in blood and heart muscles in thiamine deficient rats. In rats in which there were eloctroardiographic changes, thiamine contents decreased extremely (from 1/3 to 1/10 of normal contens). However, below the decreased level, no relationship between electrocardiographic changes and thiamine contents was observed. The authors assume that for thiamine deficient cardiac disturbance to appear it is recessary that thiamine contents decrease below a certain level (at least 1/3 of normal contents). As there was no direct relationship between degree of cardiac symptoms and thiamine contents it is assumed that cardiac disturbance is related to the degree of metabolic disturbances owing to thiamine deficiency.