The isolation of a substance very closely resembling the 18-monoacetate of D-aldosterone from the venous blood of activated muscle and from contracting muscle.

1. A renally active substance can be extracted in detectable quantity from 150 g flash-frozen active but not from 150 g flash-frozen flaccid striped muscle. This substance is identical to the 18-monoacetate of D-aldosterone (18-MA) in R(F) value in three solvent systems and in biological activity.2. The renally active substance is secreted into the blood stream by active but not by flaccid muscles.3. Bilateral stimulation of decentralized sciatic nerves causes diuresis, natriuresis, a rise in the clearances of creatinine and PAH without change in heart or respiratory rates, in cats under chloralose anaesthesia. These changes can be matched by I.V. infusion of ADH, 0.3-0.45 m-u./kg.min.4. The urinary changes caused by sciatic stimulation are not prevented by transection of the spinal cord but are abolished by hypophysectomy.5. Hence, like 18-MA, the muscle substance liberates ADH from the neurohypophysis.