1. A new spirolactone (SC 23992) has been assayed in vitro to determine its affinity for mineralo-corticoid and androgen receptors.
2. Although two to eight times as potent an anti-aldosterone agent as Aldactone in vivo, SC 23992 has only ∼10% the potency in vitro.
3. This discrepancy between potency in vivo and in vitro appears to be explained on radioreceptor assay of the principal metabolites of Aldactone and SC 23992.
4. SC 23992 may represent an antimineralocorticoid with less antiandrogen side effects.