Spirolactones are aldosterone antagonists which inhibit the binding of aldosterone to the renal mineralocorticoid receptor. These molecules also possess an antiandrogenic effect which could be due, among other possibilities, to a peripheral antagonism of androgens. This hypothesis has been tested in the present study. From in vivo experiments, spironolactone K+ canrenoate appear to inhibit the binding of [3H]5alpha-dihydrotestosterone [3H]DHT to the cytosolic and nuclear receptor of the rat ventral prostate. The doses used are in the same range as those used for demonstrating the antimineralocorticoid effect of these molecules. In vitro incubations and in vitro displacement studies show that spironolactone and K+ canrenoate are respectively about 20 and 100 times less effective than DHT in displacing 50 percent of 5 times 10- minus 10 M [3H]DHT from its receptor. Spirolactones are also able to compete with [3H]DHT for the specific 8 S cytosolic receptor. Neither spironolactone nor K+ canrenoate decreases prostatic 5alpha-reductase activity, even at a concentration as high as 10- minus 5 M. It seems likely that spirolactones, besides their action on testosterone biosynthesis, exert their antiandrogenic activity via a peripheral androgen antagonism.