The release of a renally active substance by perfused rat hearts.

1. A substance (HS) which resembles the 18-monoacetate of D-aldosterone (18-MA) in its biological actions is released by perfused rat hearts, HS is assayed in terms of activity equivalent to synthetic 18-MA.2. Isolated rat hearts perfused with modified Krebs bicarbonate Ringer without recirculation secrete progressively less and less HS.3. Addition of D-aldosterone to recirculating perfusate supplying a heart no longer secreting measurable quantities of HS causes a rapid and very marked increase in the output of HS from the heart. If radioactively labelled D-aldosterone is supplied the HS synthesized is radioactively labelled. No measurable amounts of HS are formed from 18-hydroxycorticosterone.4. Chromatographic differentiation between 18-MA and HS, initially difficult, has been achieved.5. The rate of secretion of HS from the hearts of adrenalectomized animals is markedly reduced: synthesis of HS is immediately resumed when these hearts are supplied with D-aldosterone, but at a subnormal rate.