1. Subcutaneous injection of manganese chloride caused a storage of manganese in all parts of the body but especially in the thyroid gland. 2. Injection of small doses (e.g. 0.01 mg per kg) of this salt produced an increase in the oxygen consumption of the guinea pig, while large amounts (e.g. 10 mg per kg) reduced the oxygen consumption in this animal. 3. Oxygen consumption values in the guinea pig became progressively less as the storage of manganese increased in the thyroid.