The nuclear translocation of receptor-progestogen complex was investigated in the estrogen-primed rabbit uterus. To avoid the influence of the in vivo steroid metabolism, the incubation of cytosol with progestogens, and that of the nuclear suspension with steroid-cytosol complex were performed in the in vitro systems. The quantity of the steroid receptor translocated by progestogens to the nucleus was measured using the [3H]-progesterone exchange assay. [3H]-progesterone-receptor complex in the cytosol was exchanged for cytosolic receptor sites by progesterone, dydrogesterone or norethindrone effectively. When the steroid receptors translocated to the nucleus by such steroids in the in vitro system were determined using exchange assay, progesterone translocated the receptor to the nucleus much more than did dydrogesterone or norethindrone. It was suggested that progesterone was potent in the ability of the nuclear translocation of its own receptor. The highly potent biological activities of norethindrone and dydrogesterone in vivo must be explained in the other way.