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Recently Berger et al. (1977) have demonstrated reproductive in¬hibition in Microtus montanus as a result of specific plant compounds in natural vegetation. Naturally occurring cinnamic acids and their related vinyl phenols have been demonstrated to have marked effects on uterine weight, inhibition of follicular development and cessation of breeding activity. Compounds having antigonadotrophic and antithryo-tropic activities have been identified in a wide variety of plants (Chury 1967). Bickoff et al. (1959) report that alfalfa contained a non-estrogenic compound which would over ride the estrogenic effect of the plant estrogen coumestrol. Adler (1962) demonstrated that a non-estrogenic compound in alfalfa could inhibit estrogenic responses of natural animal estrogens. Allison and Kitts (1964) extracted yellow pine needles and demonstrated that they contained a factor which would depress the uterine weight of immature weanling mice thereby delaying sexual maturation. Other investigators have reported similar findings but all failed to identity the compounds producing the effect. Gasser et al. (1963) isolated an inhibitory substance named lithospermic acid and other investigators have demonstrated it deactivates pituitary gonadotrophins in vitro.