Dihydrospirorenone (6 beta, 7 beta, 15 beta, 16 beta-dimethylen-3-oxo-17a-pregn-4-en-21, 17-carbolacton) is a new type of progestin with no other agonistic activities, in particular no estrogenic, androgenic or glucocorticoid activity. Dihydrospirorenone is a potent aldosterone antagonist 8 times as potent as spironolactone and antiandrogenic (0.3 times cyproterone acetate). The aim of the present study is to make a broad characterization of dihydrospirorenone at the receptor level and to discuss the results in comparison to those of established progestins. Kinetic studies of 3H-dihydrospirorenone uptake show a rapid increase in the amount of specific binding during the first three hours. The dissociation of 3H-dihydrospirorenone from the cytoplasmic human uterine progesterone receptor, measured by displacement of labeled steroid with dextran-coated charcoal treatment at 4 degrees C at various times, showed a monophasic or one-component, first order dissociation curve like progesterone. Sucrose density gradient centrifugation of the 3H-dihydrospirorenone-labeled myometrial cytosol showed that the dihydrospirorenone binding components sedimented in the 4S and 8S region which is typical for the progesterone receptor under low salt conditions. The high binding affinity of dihydrospirorenone to the binding sites of the mineralocorticoid receptor of rat kidney with an RBA value of 230% compared to aldosterone is remarkable. This reflects the strong antimineralocorticoid activity of this compound which was evaluated in adrenalectomized rats. Furthermore, competitive studies indicated that dihydrospirorenone also displays high affinity for the androgen and some affinity for the glucocorticoid receptors but no measurable affinity for the estrogen receptor.