Department of Zoology, University of Tennessee, Knoxville, Tennessee 37916, U.S.A. (Received 4 July 1974) The synthetic steroid R 2323 (Sakiz & Azadian-Boulanger, 1971) is being tested clinically as a contraceptive. From studies of its inhibitory action on endometrial proliferation, progesterone-maintained pregnancy in castrated animals and implantation, this trienic steroid has been shown to have strong anti-progesterone activity and weak progestomimetic activity (Azadian-Boulanger, Secchi & Sakiz, 1973; Sakiz, Azadian-Boulanger & Raynaud, 1973). Raynaud, Philibert & Azadian-Boulanger (1974) maintain that it exerts its action by competition for the corresponding hormonal receptor rather than directly impeding hormonal activity. We were interested in determining whether the mode of action for R 2323 might be found in its possible influence on the induction of uterine protein synthesis, specifically that of blastokinin (Krishnan & Daniel, 1967) in the rabbit. Blastokinin synthesis can be induced in castrated does by administration of progesterone (Urzua, Stambaugh, Flickinger & Mastroianni,