The in vitro digestibility of dry matter (IVDMD), organic matter (IVOMD) and neutral detergent fibre (IVNDFD) of pastures selected by goats or by sheep grazing on semi-arid Spanish lands were determined using rumen liquor either from goats or sheep fed lucerne hay and the effect of the inoculum source was studied. The chemical composition of the pastures varied widely (crude protein, 77.8-125; neutral detergent fibre, 404-558; lignin, 48.2-170 g per kg dry matter). Average values of IVDMD, IVOMD and IVNDFD ranged from 46.7 to 70.5, 43.7 to 70.0 and 20.0 to 47.0 per cent, respectively. Higher hemicellulose content was found in pastures grazed by sheep when comparing with those selected by goats. The phenological state of the available forage was found to be the major determinant of differences between animal species concerning the in vitro digestibility of the selected pastures. Significant linear regression equations were obtained between average IVDMD or IVNDFD and potential (PDMD) or effective (EDMD) degradability of the forages determined at grazing conditions using the nylon bag technique. It is concluded that either goats or sheep fed a standard diet can be used as donors to measure the in vitro digestibility of pastures and to predict their nutritive value when consumed by either of these animal species. Accurate prediction of the degradability of the pastures consumed by the grazing animals can be obtained from the in vitro digestibility values determined with indoors animals fed a standard diet.