SUMMARY The lactogenic effect of adrenocorticotrophin (ACTH) injected into pseudopregnant rabbits has been confirmed. A synthetic partial ACTH peptide (Synacthen) was also lactogenic. The sensitivity to local lactogenic stimuli of the mammary glands of ACTH-treated rabbits and rabbits treated with corticosteroids was greater than that in untreated rabbits. The findings are discussed in relation to the factors controlling lactogenesis.