In an attempt to relate the cardiac involvement symptoms of thiamine deficiency with biochemical changes, determinations were made of activities of pyruvate and 2-ketoglutarate dehydrogenases and levels of pyruvate, lactate, creatine phosphate, ATP, ADP and AMP in the heart, and pH, PO₂ and PCO₂ in the blood, and to correlate these with heart weight, heart rate and electrocardiogram patterns at various stages of thiamine deficiency induced by thiamine deprivation, oxythiamine treatment or pyrithiamine treatment. Pair-fed controls were used in order to rule out effects due to the inanition unavoidably associated with the deficiencies. The bradycardia and cardiac hypertrophy could not be related causally to blood acid-base changes, high levels of blood pyruvate and lactate, or deficiency of tissue energy parameters, creatine phosphate, ATP or ADP. The development of bradycardia was shown to be due to thiamine lack or antagonism in the early stages of deficiency and not to inanition. However, the marked drop in rate in the terminal stages was, to a great degree, due to semistarvation. Bradycardia was shown to persist in deficient hearts during perfusion in vitro. Most of the changes in the electrocardiogram shown earlier were found to be due to inanition. The slowing of the heart rate appears to have no cause and effect relationship to the conduction processes reflected in the electrocardiogram. The appearance and severity of both bradycardia and cardiomegaly parallel the decrease in activities of pyruvate and 2-ketoglu-tarate dehydrogenases in the heart.