1. The milk and blood iodine of a herd of thirty Guernsey and thirty Holstein cows has been determined, previous to and during prolonged increased iodine feeding to one-half of the herd. The increased amount of iodine was fed as potassium iodide which was mixed uniformly with a grain feed.
2. The blood iodine of the iodized cows was greatly and uniformly increased.
3. The milk iodine was increased. This increase was principally dependent on the amount of iodine which was fed. During late spring, however, the milk iodine from both the iodized and control cows was unusually low. During early autumn the milk iodine from the iodized cows was low.
4. The milk from the iodized cows contained 7–26 times as much iodine as that from the control cows. An average milk iodine of 80μg. % was obtained from the iodized cows throughout a mid-year period of 5 months.
5. This experiment reveals that a milk of increased iodine content can be readily produced by increased iodine feeding.