SC-11835 (1,4-pregnadiene-3,15,20-trione) is a derivative of progesterone with the ability to inhibit the renal electrolyte effects of mineralocorticoids in the laboratory. Available data indicate that the steroid is a specific antagonist of mineralocorticoids which interacts competitively with mineralocorticoids for receptor sites presumably in the kidneys. It possesses thereby a pharmacodynamic characteristic in common with spironolactone and progesterone. Quantitatively, SC-11835 is approximately equipotent to spironolactone parenterally or orally and, in similar comparisons, is 6 times more potent than progesterone parenterally. SC-11835 appears to be relatively free of other effects, as judged by data of various hormonal and antihormonal tests. (Endocrinology74: 724, 1964)