The results herein presented furnish exact critical evidence for the conclusion that the most of the milk is present as such in the udder of dairy cattle at the time of milking. The amount of milk which may be secreted during milking cannot, on the basis of these results, be over 20 per cent of the milk yield of the cow. The results show clearly that the size of the udder measures closely the amount of milk which the cow is able to secrete. The results indicate that about 1/5 of a pound of secreting tissue is necessary for the secretion of a pound of milk during a period of 15 hours. The weight of the udder during the period that the cow is dry appears to be between 6 and 8 pounds.