A full-randomized block design was used for the study of the FA composition and meat quality parameters, considering ageing time as a split-plot factor. Chemical and fatty acid composition of steaks (longissimus thoracis and lumborum muscle) from 15 month old semiextensively reared Hispano-Bretón horses were characterized (day 0), and the effect of vacuum ageing (0, 7, 14 and 21 days) on several meat quality parameters (pH, instrumental color and texture and cook loss) was determined. The average fat content of horse loin was 3.31%, and the n-3 polyunsaturated fatty acid content, although higher than in ruminant meats, suggested that the finishing on a high-grain diet limited muscle n-3 accumulation. Results revealed that ageing affected all meat quality measurements; color started to turn brownish at 14 days of ageing, with a decrease in redness but not in yellowness. Tenderness improved during the first two weeks, and the Warner-Bratzler shear force scores showed that meat aged for 7 days could be considered as ‘intermediate tender’. Under the present study conditions, an ageing period between 7 and 14 days is recommended for an optimum horse meat quality.