This work was carried out whit the aim of studying the tannins content of Erica vagans, its variation along the year and its relationship with other indicators of the nutritive value of this shrub. Shoots were harvested in Gorbeia Natural Park in May, July, August and September 2001 and January 2002. Chemical composition and in vitro gas production characteristics, in the presence or absence of polyethylene-glycol (PEG; as an indicator of tannins detrimental effects on gas production and, indirectly, on ruminal fermentation), were evaluated. Tannins content, which was high in general, was greater in the samples collected in May, with an average value of approx. 12% DM (expressed in tannic acid equivalents), did not change in July and August samplings and decreased up to a minimum value of 5.7% in samples gathered in January (P < 0.01) . The highest contents of ash and crude protein were observed in May, while contents of neutral detergent fibre, acid detergent fibre and acid detergent lignin increased throughout the grazing season (P < 0.001). The fractional rate of gas production (c) showed a tendency (P < 0.10) to be lower in samples collected in May and July and higher in those collected in January, whereas the lag time (L) showed the opposite trend , which is in agreement with the evolution of the tannins content. Neither at short nor at long incubation times, significant correlations between tannins content (determined by a chemical assay) and differences in in vitro gas production between treatments "without" and "with" PEG were observed. The best correlations between these parameters were obtained when incubations lasted for 20, 25 and 30 hours (P < 0.05).