Earlier experiments have shown that when repeated injections of a prolactin preparation are given to cows in declining lactation, a substantial stimulation of milk production occurs, though later this stimulus declines and finally vanishes, despite continued treatment [Folley & Young, 1939]. Further experiments have now been carried out to determine if the initial response would disappear with continued treatment with other pituitary preparations, and if the development of such resistance to the action of the extract was associated with the appearance, in the blood of the treated cows, of anti-prolactin [Young, 1938 c; Bischoff & Lyons, 1939] activity. Folley & Young [1938] found that the stimulating influence on milk production of cows in declining lactation of a single injection of various anterior pituitary fractions was more closely correlated with their glycotropic (anti-insulin) activities than with their prolactin1 content. It therefore seemed desirable to investigate the influence on milk production