INTRODUCTION In the preceding paper [Blaxter, 1945] it was shown that iodinated Ardein N4MB* [Pitt Rivers & Randall, 1945] had a considerable effect on the milk production and metabolism of dairy cows in mid-lactation when fed by mouth. Further preparations made from Ardein were of low potency as judged by their chemical analyses. In an experiment with dairy cows the most promising of a whole series of Ardein preparations (iodinated Ardein N9+ 10*) failed to stimulate milk production or metabolism to any appreciable extent. In doses of 50 g., these iodinated Ardein preparations produced unmistakable symptoms of iodine poisoning in one of the experimental cows [Blaxter, 1943a]. Iodinated casein preparations, however, appeared more promising from the chemical data, and experiments have since been carried out using some of the preparations described in Pitt Rivers & Randall's paper. The first part of this paper deals with the experiments which were