Lambs from two Turkish breeds, Kivircik a meat breed and Sakiz a breed used for milk and meat production, were fed a diet containing commercial concentrate and hay in the ratios of either 75:25 or 25:75. The effects on fatty acid composition were studied. After weaning (at about 8 weeks) a total of 40 male lambs (20 Kivircik, 20 Sakiz) were divided into four groups of 10 animals and fed either commercial concentrate or grass hay-based diets. The lambs were group fed indoors for 60 days. The mean intramuscular total fatty acid content of longissimus dorsi for Sakiz was lower than that for Kivircik lamb. Increasing the dried grass percentage in the ration decreased the final live weight of the lambs but intramuscular total fatty acid content increased (2088 vs. 1791 mg/kg muscle, p < 0.001). All n - 3 polyunsaturated fatty acids were higher in muscle from lamb fed dried grass-based diets than from lambs fed concentrate-based diets whereas all n - 6 were higher in the latter. Polyunsaturated: saturated ratios were higher in the latter animals; 0.26 compared with 0.16 in the lambs fed grass hay. Concentrate groups displayed a higher n - 6/n - 3 ratio in the same muscle, 7.11 compared with 1.28 in the lambs fed grass. Muscle from Kivircik lambs had higher concentrations of C18:2 n - 6 and its metabolite C20:4 n - 6 (p < 0.001) and also C18:3 n - 3. (c) 2005 Elsevier Ltd. All rights reserved.