A-norprogesterone, a steroidal homologue synthesized by Weisenborn and Applegate, was studied for hormonal and anti-hormonal activities. Testosterone-induced accessory sex organ hypertrophy in the immature castrate male rat and comb growth stimulation in the chick were partially antagonized or completely prevented by this A-norsteroid. Growth of the ventral prostate, seminal vesicles, adrenals and thyroids of intact rats and chick comb in animals not treated with androgen were significantly reduced by administration of this compound. A-norprogesterone was not androgenic, estrogenic, anti-estrogenic, progestational or anti-progestational under the experimental conditions described.