The hepatic synthesis of the androgendependent urinary protein in the rat, called a2u globulin, is strongly inhibited by estrogens. In mature male rats, treatment with estradiol-17β (0.5 μg/g body weight) completely inhibitsα2u synthesis within 6-7 days. Following withdrawal of estrogen treatmentα2u synthesis is not reinitiated for approximately 20 days. Parabiotic joining of estrogensuppressed male rats with their normal littermates within this lag period fails to change the preparabiotic pattern ofα2u synthesis in the respective partners. Besides estradio 1-17β, other estrane derivatives such as estrone, estriol and estradiol-17α were also found to inhibit the synthesis ofα2u globulin. All of the above estrane derivatives which inhibitα2u synthesis are also found to inhibit the uptake of 5α-dihydrotestosterone by the hepatic cytosol androgen binding protein of the mature male rat. Unlike cycloheximide, a known translational inhibitor, estradiol-17β does not inhibitα2u synthesis in the perfused rat liver. © 1975 by The Endocrine Society.