Previous work has demonstrated that stem bark extracts of Combretodendron macrocarpum (Barringtoniaceae), Cola nitida (Sterculiaceae), Afrormosia laxiflora and Pterocarpus erinaceus (Fabaceae) blocked the oestrus cycle of female rats through antigonadotropic activity. Moreover, a study of the plant substances responsible for these effects revealed the presence of phyto-anti-oestrogens in these plant extracts. In order to explain the mechanism by which these substances exert their antifertility actions, the interaction of the plant extract with oestrogen and progesterone receptors was studied. All crude extracts exerted inhibition of ((3)H)-oestradiol or ((3)H)-Organon binding to their respective receptors but their relative affinities were much lower than those of oestradiol or progesterone. Respective efficiencies of plant extracts in competing for the oestrogen receptor were as follows: A. laxiflora>> P. erinaceus>> C. macrocarpum>> C. nitida. The efficiency order of competition for the progestin receptor was different to that of oestrogen. The most potent competitor was C. macrocarpum extract, followed by P. erinaceus, C. nitida and A. laxiflora. Moreover, the interaction between oestradiol and plant extracts with the oestrogen receptor was determined to be competitive only for C. macrocarpum and A. laxiflora, whereas all compounds produced a competitive inhibition on the progestin receptor binding. These results suggest that the plant extract binding site was the same site as for the steroid. These results suggest also that crude plant extracts may interfere with natural oestrogen and/or progestagen in vivo by binding to steroid receptors.