The purpose of this study was to examine the fatty acid and chemical composition of the Longissimus dorsi (LD) from male Tuj and Hem??in lambs reared in extensive, semi-intensive and intensive feeding systems. At the end of 90 days eight lambs from each breed and feeding system were slaughtered to determine chemical composition, and six lambs in each group were selected at random to assess fatty acid composition. Breed and feeding system interaction affected the quantities of saturated fatty acids (SFA), monounsaturated fatty acids (MUFA), and polyunsaturated fatty acids (PUFA), and the atherogenic (AI), thrombogenic (TI), and nutritive value (NVI) indices. In Hem??in the ratio of PUFA to SFA was higher in lambs fed in the extensive system than those produced in the semi-intensive and intensive systems, which were similar, whereas in Tuj this ratio decreased from the extensive to semi-intensive to intensive feeding systems. The ratio of omega n-6 to omega n-3 fatty acids was lower in the extensive and semi-intensive systems than it was in intensively fed Hem??in lambs, but increased with the intensity of feeding in Tuj lambs. Intramuscular fat content was higher in Hem??in lambs than in Tuj lambs and increased with the intensity of the feeding system. Conjugated linoleic acid content (CLA) was affected by feeding system in Hem??in lambs, but not in Tuj lambs. Because of their high PUFA/SFA ratio and low TI value, Tuj lambs reared in extensive feeding system were deemed to have the best performance.