In the previous paper [Folley & Young, 1945] we showed that cows of the breeds we have employed and under the conditions of our experiments respond regularly to the administration of anterior-pituitary extract during the period of declining lactation, an increase in total milk yield of about 20% above that expected in the absence of treatment being obtained when a suitable dose was administered at 2-day intervals over a period of 3 weeks. Although during the 2 weeks immediately following cessation of treatment the mean yield was still above that expected, the effect was not sustained throughout the period of lactation and the daily yield ultimately became similar to that which might have been expected had pituitary treatment not been instituted. It seemed to us possible that if, instead of cows in declining lactation, animals before or during the peak of lactation were similarly treated, it might prove possible to