Subterranean clover (SC) has been found to contain oestrogenic compounds, such as genistein (G), biochanin (B), and coumestrol (C), which are extensively metabolized in the rumen by the microbial population. To verify if these substances or their metabolites can interfere with the rumen degradation of feeds, an extract from SC was prepared, purified, freeze-dried and utilized for in vitro trials of rumen degradability on several feeds: wheat straw (WS), cocksfoot hay (DH), lucerne hay (LH), barley grains (BG) and soybean meal (SB). The feeds were incubated for 48 h in rumen fluid + McDougall buffer with and without the addition of 10 pg/ml SC extract (N 6 6 both for the treated and untreated samples); the degradation percentages of organic matter (OM), NDF and crude protein (CP) were calculated from the chemical analysis of the residues. The trials were repeated using synthetic oestrogens, G (N = 4), B (N= 4) and C (N= 4).Subterranean clover extract always enhanced OM degradability, but the effect was influenced by the chemical composition of the feeds, and was larger for the high-protein feeds (SB = +4.6% with a significant difference, P 0.001 and LH = +2.5%, P < 0.01) and smaller for the fibrous feeds (BG = +0.5%, DH = 1.2%, P <_ 0.01 and WS = +1.3, P <_ 0.01 ). G showed a similar effect as the SC extract (7.2%, P < 0.001 and 4.7%, P <_ 0.001 for SB and LH respectively), while B and C did not significantly affect OM degradability. SC extract and G also increased the degradability of NDF of SB (5.9%, P <_ 0.001 and 10.0%, P < 0.001 ), LH (3.4%, P <_ 0.001 and 4.8%, P < 0.01) and DH (1.2%, P<0.05 and 4.1 %, P < 0.001 ); their effect on WS and BG was not significant.