ABSTRACT The presence of oestrogen inhibitory activity in oat hay (Avena sativa) and Fahli clover hay (Trifolium alexandrinum var. Fahli) has been established. The antioestrogenic effect was demonstrated by the inhibition of uterine weight increase in rats (Astwood test) in response to oestradiol injected together with the above mentioned plant extracts. The extraction procedures are described in detail and the possible biological implications of antioestrogenic and oestrogenic activity in fodder plants is discussed.