The problem of what proportion of the milk normally obtained at a regular milking is already free in the udder when the milking begins, was attacked by milking 3 cows at regular 12-hr. intervals, determining the weight and lactose content of the milk, and finally killing the cow (unmilked) at a regular milking time. The milk and tissues of the amputated udders were analyzed for lactose. The methods of preparation and analysis are described. On the basis of the average lactose yield at the regular milkings, the percentage recovery upon slaughter for the 3 cows was 100, 96 and 118. The last result is regarded as the most reliable on account of improved technique of udder analysis. The authors infer from these data that there is present in the udder at milking time an amount of milk in excess of the amount secured upon milking. Very roughly, 1/4 of the milk is obtainable by milking the amputated udder, another 1/4 by drainage from the sliced udder, while the remaining 1/2 is tenaciously held even in the finely ground tissues. Previous experimental work is not in serious conflict with these results. A general theory of the physiology of milk secretion is outlined as a working hypothesis.