A series of experiments designed in accordance with the biogeochemi-cal hypothesis on the etiology of Keshan disease was performed. It was shown that the pathogenic factors of Keshan disease existed in the grains cultivated in the endemic regions ("endemic regions" mean those localities where Keshan disease has been prevalent). These factors had an ability to elicit a metabolic derangement of calcium in the myocardium and indu- ce some latent damages in myocardial cells. Furthermore, there were definite relations between these factors and the nitrite. With the aid of nitrite, more serious myocardial necroses were produced by these pathogenic factors. It was also proved that the addition of ammonium molybdate to the grains from the endemic regions produced some prophylactic effect in reducing the myocardial injury in expermental animals. After an observation on fertilizing the soil in endemic region with ammonium molybdate for four years, the preliminary result showed that this measure seemed to be effective in preventing Keshan disease.Based on the results cited above, we considered that molybdenum deficiency and nitrite excess might play an important role in the occurrence of Keshan disease, but the exact pathogenic factors in the grains remained to be studied further.