One of us1 showed that adrenalectomized rats could bear young if treated with cortin. No attempt was made to raise the young. Carr2 found that cortical extract made according to the Swingle and Pfiffner method did not support lactation. Swingle and Pfiffner3 have recently shown that their extract will support lactation in the dog.Extract made according to our method which removes fatty substances by chilling to —12°C. does not enable adrenalectomized rats to supply milk (B, Table I) even when used in large amounts (C), yet it is potent as far as the cortin content is concerned. On the other hand, if the chilling is not carried below 3°C, our extract supports lactation (D). The material chilled out between 3 and —12°C. contains a substance necessary for milk production, since if an extract of this material is added to cortin and injecte into adrenalectomized mother rats, they raise a much larger proportion of their young (E).The evidence indicates the existence of a new hormone in the adrenal cortex which ...