Twenty-two samples (leaves and flowers) collected from six Mediterranean shrub species have been analysed regarding total extracted phenols (TEP), extracted tannins (ET) and condensed tannins, using either the butanol-HCl (TCE-but) method or the vanillin acid (ECT-van) method. The biological activity of phenolic compounds was studied, determined as the effect of polyethylene glycol 6000 (PEG 6000) on in vitro digestibility (IVD) and gas production was measured after 24 and 48 h of incubation in rumen juices. A weak correlation was observed between TEP and TCE-but-TCE-van. However, significant and high correlation coefficients were found between TEP and ET, on the one hand, and between TEC-but and TCE-van, on the other. A positive and significant correlation (P less than0.01) was established between tannins and their biological activity measured as the effect of PEG 6000 on gas production.