The formation of a renally active peptide by cat lungs from gamma-globulin in vitro and the plasma concentrations of this peptide, in vivo.

1. A substance (LS), extracted in aqueous acetone and separated by partition and thin-layer chromatography, has been obtained from lung perfusates and from the systemic blood of cats.2. LS is assayed biologically in rats during sustained saline diuresis by measurement of the muequiv Na retained. This parameter is linearly related to the log concentration of LS infused.3. Lungs perfused with Tyrode fluid neither contain nor produce LS. Production of LS begins when gamma-globulins (in cat plasma, mixed ox globulins or equine Cohn II) are present in the perfusing fluid.4. The proteinase inhibitor trasylol does not prevent the formation of LS by lung; trasylol protects LS from break-down during re-perfusion and during isolation from perfusates or from blood.5. In cats under chloralose anaesthesia LS is present during haemorrhage in systemic arterial and mixed venous blood, the arterial concentration of LS is approximately four times the venous.