Long-Evans rats hypophysectomized on the 12th day of pregnancy showed a small amount of milk in their mammary glands on the day of parturition. Untreated, such rats resorbed this milk and were unable to supply enough to their own or foster young for even one day's maintenance. Mammotropin (MH) and cortisol proved to be adequate substitution therapy in raising the milk production in such animals to approximately 50% of normal, and in maintaining it at that level. During these tests the mothers lost about 1 gm. of body weight daily while each of six pups gained that amount. It was unnecessary to inject oxytocin for milk conduction in these rats, although the degree to which the endogenous secretion of this hormone had returned was unknown. No good evidence was obtained to prove that either somatotropin or thyroxin, when added to an MH and cortisol regimen, improved the lactational response. No one of the mentioned hormones supported lactation nor did any combination that excluded both MH and cortisol. These two hormones are not only important in initiating milk secretion, but also in maintaining lactation in the rat.